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PostPosted: Wed Jul 08, 2020 10:26 am 
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Hey guys - hope everyone is doing OK and staying safe!

Ever since I pulled my '80 5speed (N/A) out of storage of few weeks ago, I've been struggling to diagnose an issue I had never experienced with it before.

Once the car is fully warmed up (15-20 minutes of vigorous driving), the engine begins to buck, can't get power or rev the car in anything higher than 2nd. Sometimes the gas pedal has absolutely no effect, once or twice it has even backfired from what appears to be under the hood. The car is not overheating - the temp gauge rarely hits even the half-way point and the coolant level and condition look great.

So, here are all the things I've done so far:

1. FUEL: Because the car had been sitting for at least a year, perhaps 2 (I don't recall), I knew the gas was more than stale. I pulled the fuel line right after the fuel filter and used the fuel pump to get most of the bad gas out the tank. I removed the gas pickup assembly from inside the hatch area to inspect the tank and the mesh filter on the pickup. Both looked spotless (clearly someone had refinished the tank).

I installed a new fuel filter and a fuel pressure gauge right after the filter and the results under no load are within spec (low 30's) and goes up by 6 psi when I pull the vacuum hose from the fuel pressure regulator. The only time I can hear the fuel pump making a particularly loud whine is when that FPR hose is off. I also checked the voltage at the pump and it gets 11.9 volts while cranking (with the starter wire pulled to prevent starting).

For the heck of it, I also swapped the gas cap with a another ZX cap just in case there was a venting issue, but that seemed to have no effect.

2. VACUUM: I put a vacuum gauge on the line going to the brake booster cable and I get about 12-13psi. That's low, but it might be explained by the fact that the engine has been modified (according to the PO: head port + polished, milled for compression; competition valve job, custom ground cam (Schneider) with matching springs). I searched high and low for vacuum leaks using carb cleaner, found and repaired one minor one. Also found a gaping hole in the boot that goes from the K&N filter tube into the AFM and took care of that too.

3. ELECTRICAL: I cleaned every electrical connection and relay under the hood as well as the ECU and the ECU harness. I then ran through all the diagnostic checks with my multimeter on the ECU harness that the FSM suggests. Everything was within spec except for the thermotime switch, which appear to be bad but that shouldn't affect anything other than cold conditions. I pulled the distributor cap and cleaned the cap and the rotor, although both looked fine.

4. AIR INTAKE: I wanted to rule out the AFM, so I opened it and cleaned the carbon track, which looked good. Both the AFM and the throttle body looked nice and clean inside. When I tested the AFM's resistance as I opened the flap/vane, I noticed the resistance jumped around a lot, sometime going lower even as the vane opened more. I then ran a long wire from the AFM to my multimeter on my dash so I could watch it as I drove. I monitored the volts as I changed speed and acceleration. It seemed to be giving values I expect - the voltage change was in line with the vane opening and closing.

5. ENGINE: I pulled the plugs and all six look about the same and seem OK. But that's not too surprising because my problem is only very recent. I then did a compression test on all 6 cylinders and all were very close in value (between 135-142). For the heck of it, I also replaced the PCV valve. I listened to all six injectors while the car was running - injector five was clicking a little quiet compared to the others, but the rest seemed fine. Ran about 5 ounces of SeaFoam through the brake booster vacuum line and also put some in the tank with the fresh gas but that did not solve the problem.

This is a California car, so it doesn't have a water temperature sensor going to the ECU (although it does have an O2 sensor, long since disconnected). So, the ECU relies solely on the CHTS for temp. info so I pulled it and confirmed it is working perfectly.

So, the only thing I can think of at this point to do next is to run the fuel gauge out of the hood so I can monitor fuel pressure it while it's under load. I suppose rather than just die completely and make it the obvious problem, perhaps the fuel pump no longer has what it takes to keep up the pressure once it gets hot and is pushed under load. Has anyone ever heard of this happening?

Special thanks to helpful neighbor and all-around good guy johnnyz for allowing me to recruit him to take a test drive and spend a morning together under my car!

I'd love any and all suggestions or insights! It's a pain not to be able to enjoy driving the car right now, but on the bright side I'm learning so much and the car is definitely running better than ever until it heats up and struggles.

Thanks!

- Jim


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PostPosted: Wed Jul 08, 2020 10:45 am 
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Location: rhode island
Pull the TPS (throttle position sensor,) plug of the sensor and then see how it does. If you have a Cylinder Temperature sensor in between spark plus 5-6 then check the connection. Ohm meter to check sensor form a cold start to warm to see if it's working. (you might not have one, but just saying if you do.) Then last but not least there is the MAF sensor adjustment. You have to remove the lid that is glued down. My 81t was a Dog when I got it home, probably why the guy sold it. After I adjusted the Air Flow Meter it woke right up. Finally at this age you need to pull every plug off all computer related sensors, even the computer, and carefully file all connections. Green corrosion you can see, but the fine grey corrosion you can't. Well, at least I can't. :shock: Also, make sure there is no fuel in the vacuum hose to the fuel pressure regulator. There is also the little screen on the fuel pump that can get clogged. Also, did you know 1980 is a special transmission? I've been looking my entire Z life for one. It should be a close ratio like in all N/A ZX's from 81-83, but 1980 has a taller 5th gear. My last parts car was a 1980, I was so excited..........only to find out it was swapped out for an older 5 speed, what a let down. :cry: < not enough tears.


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PostPosted: Wed Jul 08, 2020 11:33 am 
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Thanks for the tips, Paul!

Paul wrote:
Pull the TPS (throttle position sensor,) plug of the sensor and then see how it does.

I checked the resistance of the TPS at idle, at WOT and at in between, and all three showed continuity where and as expected both on the TPS itself and on the ECU wiring harness. Should I pull the plug and then drive it to see whether the problem still occurs?

Paul wrote:
If you have a Cylinder Temperature sensor in between spark plus 5-6 then check the connection. Ohm meter to check sensor form a cold start to warm to see if it's working. (you might not have one, but just saying if you do.)

I not only read the resistance at cold and hot while installed, I also pulled it and tested it in hot and cold water. Both times it passed.

Paul wrote:
Then last but not least there is the MAF sensor adjustment. You have to remove the lid that is glued down. My 81t was a Dog when I got it home, probably why the guy sold it. After I adjusted the Air Flow Meter it woke right up.

I opened the MAF/AFM casing and cleaned the carbon track but did not mess with loosening the screw and moving the gear because I didn't see why that would all of sudden need to be adjusted if the car ran great when I put it into storage and has run poorly since the moment it came out.

Paul wrote:
Finally at this age you need to pull every plug off all computer related sensors, even the computer, and carefully file all connections. Green corrosion you can see, but the fine grey corrosion you can't. Well, at least I can't. :shock:

Used electronic contact cleaner, toothbrush, wire brush and emory cloth where safe and possible.

Paul wrote:
Also, make sure there is no fuel in the vacuum hose to the fuel pressure regulator.

Yes, no fuel nor any smell of fuel. FPR does not appear to be leaking at all.

Paul wrote:
There is also the little screen on the fuel pump that can get clogged.

I was unaware that the ZX's have a screen right before the pump, I thought that was only on the earlier models. I thought the only pre-pump filter on the ZX's is the mesh around the gas pickup in the tank, which I checked and confirmed was spotless. Please tell me how I can access that screen - do I simply pull the feed line that goes into the pump and see it on the pump inlet? Given how the electric contacts on the pump seemed to be laminated in place with some plastic substance, I assume this is the original, factory-installed pump, so if the screen was there from the beginning, it's probably still there.

Paul wrote:
Also, did you know 1980 is a special transmission? I've been looking my entire Z life for one. It should be a close ratio like in all N/A ZX's from 81-83, but 1980 has a taller 5th gear. My last parts car was a 1980, I was so excited..........only to find out it was swapped out for an older 5 speed, what a let down. :cry: < not enough tears.

I did not know that! What's the advantage of having a taller 5th gear?

- Jim


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PostPosted: Wed Jul 08, 2020 12:09 pm 
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Joined: Wed Apr 09, 2008 5:53 pm
Posts: 14779
Location: CT
I don't know the ZX cars all that well, but I might add these thoughts ~

In any experiment (automotive searches especially), heat is always a factor of change.

1. I have known drivers whose cars ran well at start up and had full cylinder pressure, but when the car got hot the head gasket leaked.

2. Another driver with an original 6-cylinder dual-carb Corvette ran well cold, but when hot one of the carbs percolated.

3. My own simple Z has twice suffered from an ignition coil which broke down at high temps and could make me stall under some conditions.

4. Another CTZCC member forgot to open his Summer/Winter airflap and his car refused to run well when hot.

5. My teenage bud's '55 Chevy ran well until he reached a speed which moved a loose primary ignition wire into contact with an unintentional ground, at which time the car sputtered and often died. That took weeks to find.

6. My first Murano (V6) had a variable cam timing problem which often stuck the valve timing in constant advance when the car reached OpTemp, killing power and fuel economy (a $139 part which would have cost $2,000 to reach).

7. Another Clubbie with a highly modified 240 claimed his like-new ignition computer broke down under heat; he replaced it and the problem was solved.

Another member's ZX got a mouse nest in the intake over the winter, and parts of the mouse and nest got pulled into at least one cylinder.

9. Other considerations regarding breaker point ignitions wouldn't apply to your electronic ignition.

FT

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1970 240Z


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PostPosted: Thu Jul 09, 2020 7:58 am 
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Location: rhode island
The ZX can run good with the TPS disconnected. If there is any condensation in the plug it will run like crap. For example: I just pressure washed my 81t engine bay, and I always do this while the car is running. Did this to alot of ZX's in my time. The only thing is water gets in the TPS connector and it will not go over 2000 RPMS. Unplug it and it runs fine. Once plug is dry I plug it back in and all is good. So as far as the AFM, the screw that holds the wheel: Carefully loosen screw while holding the wheel, or else it will go twang, no big deal it wont damage nothing, just put it back if it does. Now you let some tension off the wheel, about 3-5 teeth. These are the 2 very common issues. I can's explain the why about the MAF, but it works. Dont worry to much about the screen in the pump, last case scenario. Just pull the fuel line off the engine side of the fuel filter, put it in a jar, turn key on and see if shoots out in a nice stream and not trickling. (Redneck way:pull hose off top of filter, turn key on, gas should shoot about 10 feet high.) :lol: Also a key note: If car runs good cold, computer is in open loop relying on preset perimeters. When the car runs like crap at operating temperatures, computer is in closed loop relying on sensors. MAF, TPS, CHTS, CTS, IATS. So you could have a bad coolant temp sensor for the computer, not the temp sensor for the gauge. The intake air temp sensor is located in the front of the MAF.


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PostPosted: Thu Jul 09, 2020 8:27 am 
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Joined: Thu Oct 08, 2009 1:14 pm
Posts: 244
Location: Southington, CT
Hi Jim - I had a similar issue with my 1980 280zx. It was a black/red anniversary car that I bought sight-unseen, so I had not had the opportunity to drive it before purchase. When it showed up, it would barely get out of it's own way when trying to drive it... it would bog, pop, and seemingly backfire from under the hood when I tried to engage it in first gear. Luckily, I have an excellent shop near my house that diagnosed/alleviated the problem, which was a bad Boost Control Deceleration Device (BCDD). I don't remember the specifics, but apparently there are parts inside them that go bad (rubber, I think?), and I was told that new / remanufactured ones are are impossible to find. ..so they did some kind of work-around fix, which made the car perform great (I think they blocked something off).

Don't know if that's your problem, but it might be worth looking in to.

All the best!


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PostPosted: Thu Jul 09, 2020 12:27 pm 
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Location: rhode island
Yes that could also cause the issue, usually it will run bad all the time, but something to look into. It's one of those things I would just remove and block off, like EGR valve and the AAC valve if equipped. If it runs great cold and falls on it's face when warm, it's usually computer, sensor related. BTW to answer the 5th gear question, lower RPM's at highway speeds and might go a little faster at top end speed. It's only slightly taller, so nothing dramatic over the other years.


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PostPosted: Thu Jul 09, 2020 7:47 pm 
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Thanks for the suggestions, guys. I'll definitely look into the BCDD.

Today I drove with a fuel pressure gauge on my windshield so I could see exactly what was happening to the fuel pressure when the car is running right (first 10 minutes) versus when it's struggling.

When it's running well, the fuel pressure goes up as I accelerate but once the problem starts arising, accelerating no longer leads to higher fuel pressure, in fact, the fuel pressure gradually creeps lower as I drive, even if I'm coasting, sometimes hovering around 20psi.

Another difference I discovered between when the car is OK and when it's not: when it's idling before it's even been driven, pinching the fuel return line spikes the fuel pressure up to 50 psi as expected. When I get back to my garage and the car is no longer running OK, pinching the return fuel line has no effect - the car just stays around 30 psi.

If the fuel pressure is not changing when the return line is pinched, does that mean that the pump is just too exhausted to do any more than it's already doing or is there another possible explanation given that fuel is not spraying out? If the fuel were simply dumping into leaky injectors, why would the car be smoothly idling just as it was when it was first started? Or could this be a sign the fuel pressure regulator is defective? Is it even possible for the FPR to malfunction in a way that would LOWER fuel pressure given there's no sign of gas going into it's vacuum hose?

Thanks so much - your help is greatly appreciated!

- Jim


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PostPosted: Fri Jul 10, 2020 9:17 am 
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Well I didn't know you had a fuel gauge, so that would be the correct way versus the redneck way. So I believe it is narrowed down to a fuel issue. Just because there is no gas in the pressure regulator vacuum hose, it can still be bad. I would just install a new one since it is a very easy thing to do. Then that will rule that out if problem persist. I never encounter a ZX with a bad fuel pump, even at 300,000 miles. Not saying it could be the fuel pump. There is also a pulse regulator, (I think that's what it's called,) right next to the fuel pump. That could be an issue too. If you have compressed air, try blowing some air back through the fuel pump. Do this to both the feed line and return line. Take the gas cap off when doing so.


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PostPosted: Fri Jul 10, 2020 12:46 pm 
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Paul wrote:
Well I didn't know you had a fuel gauge, so that would be the correct way versus the redneck way. So I believe it is narrowed down to a fuel issue. Just because there is no gas in the pressure regulator vacuum hose, it can still be bad. I would just install a new one since it is a very easy thing to do. Then that will rule that out if problem persist. I never encounter a ZX with a bad fuel pump, even at 300,000 miles. Not saying it could be the fuel pump. There is also a pulse regulator, (I think that's what it's called,) right next to the fuel pump. That could be an issue too. If you have compressed air, try blowing some air back through the fuel pump. Do this to both the feed line and return line. Take the gas cap off when doing so.


Thanks for the suggestions, Paul - I'll definitely try them.

Here's another piece of info that is really puzzling: As I already mentioned, after driving the car, when I pinch the return line there is no change to the fuel pressure. However, if I turn off the car and immediately restart it, pinching the return line once again pushes the fuel pressure up beyond 50psi. It's like something is getting reset simply by the car turning off and back on. Perhaps restarting the car removes a fuel system vacuum, or releases a stuck check valve or clog, or something like that? Clearly something is happening just by restarting the car that returns the fuel system to normal operation. Whatever that is, it's probably the culprit of all of my problems.

- Jim


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PostPosted: Fri Jul 10, 2020 3:21 pm 
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** UPDATE: I swapped in another FPR, but no luck - exact same problem. :(

Also, when I had the FPR off, I removed the gas cap and ran compressed air through the FPR's return line and it went through with no resistance.

And again I confirmed that shutting off the car and immediately restarting it causes something to reset because the fuel pressure problem went away until something builds up again after 5-10 minutes of working under load.

- Jim


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PostPosted: Fri Jul 10, 2020 4:36 pm 
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Well that's one thing eliminated. Technically you could pull and replace each item with a known-good replacement and eventually that would solve your problem.

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PostPosted: Sat Jul 11, 2020 9:24 am 
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A simple FPR is one thing but we want to avoid replacing everything if it doesn't need to be. If I could just put my hands on the car I would figure it out with the power of the ZEN. So the key is the fuel gauge you have hooked up. So you are saying it's dropping in pressure and that's when it acts up. Just a thought, try running it with the gas cap off. In the mean time I will see if I have a factory manual, do some research on this and see if something jars my mind. Also listen to the fuel pump when it is running good and when it starts running bad, does it change it's tune. A long screw driver with the tip on the pump and put the handle up to your ear if you can't hear it over the exhaust.


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PostPosted: Sat Jul 11, 2020 9:31 am 
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Thanks, Paul!

I listened to the fuel pump - it doesn't seem to be making a different sound based on how its running. I do know it's getting pretty hot to the touch - is that normal? Also, did the stock 1980 fuel pump have an inlet filter or does it just rely on the mesh canister in the tank's pickup assembly?


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PostPosted: Sun Jul 12, 2020 2:01 pm 
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Did you run it with the gas cap off? This will determine if the carbon canister is working. If not it could be building up pressure in the tank causing a restriction on the return line. I only found my 81 Turbo factory manual, and to test the pump it should be about .5 to 3 ohms on the positive and negative power at the pump. Unplug it to do the ohm test. The plug is under the back carpet mat, and there's also a round access hole to get the sending unit out, or to look in the tank. Do the Ohm test when it is cold, then once it runs for a wile and starts acting up, do the ohm test immediately. I don't believe the pump should get hot. They all should have that screen, but hard to remove when attached underneath. Easy on the bench, but at that point might just want to install a new pump. They have them at Motorsports for all non-turbo Z's. Just to confirm you had the fuel pressure gauge hooked up in between filter and feed line at the rail, and should be 30 psi at idle and 37 psi when you rev it up when it was running good. Then it dropped and ran bad. It could very well be the pressure relief valve is bad in the pump, so pump seems to run fine, but it's not. Try the gas cap idea first just to rule that out, not common but easy to do. Also pulse regulator could be bad, but I don't see any test procedure for that. No adjustment either. No availability either. I would just look through and make sure it is clear through. I would say if you put your finger over one end and compressed air on the other if you can turn the air down to 35 psi, then blow through should hear the spring move. As far as I can see here, you either have a clog somewhere's or the pump itself even though it's not common, it can happen.


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