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Discussion Starter · #1 ·
I am reconditioning a 1987 A/T Z31 NA and have hit a wall and am looking for help. The current problem is that the engine seems to be misfiring and idling poorly. My dad and I have done a lot of work to it including valve/head work, new headers, new ignition coil, plug wires, valve cover and head gaskets, vacuum lines, new brake master cylinder, Thunderbird electric fan conversion, etc.

Running the car and switching to real-time diagnostic mode V doesn't show any errors. In mode III we get an error code for the cylinder head temperature sensor. We've used the factory service manual (FSM) to test the ignition coil, crank angle sensor and fuel injectors. I've also checked fuel pressure before & after the filter. All tests pass and do not indicate faulty parts/sensors.

The timing belt was installed per FSM. Although ignition set for 20 degrees BTDC, the timing seems to have drifted to closer to 30 degrees BTDC when using a timing light. I've also connected the timing light to other spark plug wires and they seem to be missing signals based an inconsistent/irregular light strobe at times. Some cylinders seem worse than others. Another odd thing is that cylinder bank 1-3-5 gets noticeably hotter than the bank for cylinders 2-4-6. At this point I've checked the things I can think that may be causing the problem, but still have been able to identify the source.

Just recently we made a trip to the junkyard and tried swapping the crank angle sensor and its accompanying disc, as well as make the upgrade to a PRW-2 power transistor, but the changes had little to no effect on the running condition. I've included a video of a recent start - the car starts quick and strong but after a few seconds it runs extremely rough, often stalling itself out at idle.

Any thoughts/suggestions are welcome.
 

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From what I can hear and see, the car starts quickly and smoothly. Around the 15 second point in the video the engine starts to stumble and heads down hill.
As the "start systems" are shutting off something is going wrong

There may be a timing problem, and you should figure that out and fix it, but I don't believe that is causing your car to stall out.

This is most likely a fuel or fire problem. If the cas is bad, that cases timing, fuel, and fire issues.
What is failing the fuel injectors or the spark plugs?
With no other information, I would want to watch a fuel pressure gauge during the start/idle/stall process.

After a start/stall, as shown in video, will the car start again if the key is turned off then on, pause for 5 seconds, then start? Is the second run different?
What is the behavior if you open the throttle?
 

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Discussion Starter · #3 ·
Just got around to making a new post.
I haven't been able to replicate the 'starts good then bogs down' issue since I made my first post. The current trend recently seems to be that the car will take a few starts and subsequent stalls before idling at a reasonable rpm (around 800) and running fine. Where it gets tricky is that sometimes the engine will not hesitate when I give it gas and other times it will. I was confident enough to take it for a short drive recently when the weather wasn't too cold out, and the car seemed to alternate between hesitating when applying throttle and not hesitating, during driving.

My dad also just recently purchased a Picoscope to take a better look at what the electronics were doing. From a simple first test of connecting it to the plug wire going from the coil to the distributor, it seemed like spark was being delivered consistently - EXCEPT for an intermittent 'miss' where inadequate or no voltage at all was supplied - while the engine was idling. I have yet to test and see how the spark looks when giving it gas.

Yesterday when it was colder out I tried to drive it down to the gas station to fill the tank but the engine was running very poorly - acceleration was incredibly slow and the hesitation was horrendous.
From what I've gathered I think the problem lies in spark but it's still confusing to me as the coil and wires are relatively new.
Thanks in advance.
 

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Discussion Starter · #4 ·
Quick update on the issue -
Took a look under the hood with my timing gun and found that the timing was off from the marks on the crank pulley. I adjusted the distributor according to it's correct timing position (around 26 degrees according to the notches if I remember correctly). After which, it ran noticeably better and had no hesitation when revving. After a short period of idling however, I noticed that the idle condition worsened. I checked with the timing light again, and the timing had shifted back to the incorrect position (at around 30 degrees BTDC). The distributor hadn't moved at all, so I am unsure of why the timing is changing itself.
Blue Product Liquid Glass Light

This is a still from a video I took of the timing after it shifted itself over, while checking it with the timing gun. The red circle indicates where the timing is now (30 degrees), while the blue indicates the correct position to where it had been adjusted to previously (around 26 degrees).
Any insight helps.
 

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I would guess the ECU is getting confused at to the state of the engine.
I would start by making sure your cars power and ground connections are good. Make sure the ground connection from the battery to the chassis is in good shape with clean tight connections.
If the power and grounds are good, run the ECU's self diagnostic. I would guess as the engine is warming up, a temp sensor is not working, but that should not cause a timing jump.
Also check the all the connections for the ECU system.
 

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I've not ever seen the timing shift issue and I feel it's the first thing to straighten out. Is it possible the woodruff key is missing from the crankshaft snout?

Also, you were suspecting a spark issue, and I can see how that would be confusing given what you've replaced already, but one thing I've had much trouble with is cracked spark plug insulators (the white part) and arcing. I would check each spark plug carefully for any cracks. And make sure dielectric grease is in all the right spots on the ignition wires to prevent any arcing.
 

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I've not ever seen the timing shift issue and I feel it's the first thing to straighten out. Is it possible the woodruff key is missing from the crankshaft snout?

Also, you were suspecting a spark issue, and I can see how that would be confusing given what you've replaced already, but one thing I've had much trouble with is cracked spark plug insulators (the white part) and arcing. I would check each spark plug carefully for any cracks. And make sure dielectric grease is in all the right spots on the plugs wires to prevent any arcing.
I was thinking the same thing
 

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I am reconditioning a 1987 A/T Z31 NA and have hit a wall and am looking for help. The current problem is that the engine seems to be misfiring and idling poorly. My dad and I have done a lot of work to it including valve/head work, new headers, new ignition coil, plug wires, valve cover and head gaskets, vacuum lines, new brake master cylinder, Thunderbird electric fan conversion, etc.

Running the car and switching to real-time diagnostic mode V doesn't show any errors. In mode III we get an error code for the cylinder head temperature sensor. We've used the factory service manual (FSM) to test the ignition coil, crank angle sensor and fuel injectors. I've also checked fuel pressure before & after the filter. All tests pass and do not indicate faulty parts/sensors.

The timing belt was installed per FSM. Although ignition set for 20 degrees BTDC, the timing seems to have drifted to closer to 30 degrees BTDC when using a timing light. I've also connected the timing light to other spark plug wires and they seem to be missing signals based an inconsistent/irregular light strobe at times. Some cylinders seem worse than others. Another odd thing is that cylinder bank 1-3-5 gets noticeably hotter than the bank for cylinders 2-4-6. At this point I've checked the things I can think that may be causing the problem, but still have been able to identify the source.

Just recently we made a trip to the junkyard and tried swapping the crank angle sensor and its accompanying disc, as well as make the upgrade to a PRW-2 power transistor, but the changes had little to no effect on the running condition. I've included a video of a recent start - the car starts quick and strong but after a few seconds it runs extremely rough, often stalling itself out at idle.

Any thoughts/suggestions are welcome.
 

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Dude, This sounds exactly like what was going on with my ‘88. Too make a long reply short, I don’t know why exactly this was the problem but I replaced my battery and my car ran and is running now better than I can ever remember. I think I had a dead cell.
 

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Discussion Starter · #10 · (Edited)
For clarification, I've now gotten it to idle fine as long as I give it some time to cycle the engine and warm up.
But, whenever I try driving it at a reasonable speed the car bogs down horribly and loses power. Previously, I tried taking it out for gas and it barely made it out of the neighborhood before I had to turn back because it was behaving horribly (hesitation, cutting out, etc).


I've since adjusted the timing to a correct position by moving the distributor and made sure to tighten down the locking bolt sufficiently. I went and attempted to drive it again just yesterday to get an idea of what was happening with it, seemed fine with little hesitation while I was driving inside of the neighborhood... as soon as I tried to get it up to the speed limit (35mph) it started running very rough. It was cutting out, hesitating and misfiring loudly (I have an exhaust leak at the stock cat), managed to limp it home. It could hardly rev past 2000 rpm and was WAY down on power. Could hardly make it to 30mph. Decided to check the timing once I got home and it was back at that white mark (should be at the orange mark)! Although, another z31 owner told me that the timing wouldn't be causing it to run how I described it, and that it may be a MAF issue. Still pretty lost with getting this thing to drive fine.
108165

Any advice is appreciated.
 

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This is a shot in the dark, but I'll try it.

Your car is running as badly as my '83 was. I disconnected the battery, then completely disconnected the ECU for 30 minutes, re-installed both, and it ran great. It was essentially a hard reboot of the ECU. (Idea suggested by zmanofwashington.) It's free to try it.

That and vacuum leaks accounted for my rough running problems.
 

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Your post mentioned a CHTS error from the ECU. Have you fixed this? As your engine warms up, the ECU needs to lean out the fuel/air mix. The CHTS signal is used to do this.
 

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I totally agree with thansen above. If your CHTS is nfg your engine will absolutely not run right. Check it via the FSM. Also the exhaust gas sensor on my Z failed recently, but the ECU did not give a code. It would start and idle ok when cold, but once the engine got warm, it wouldn't idle worth a darn. That can also be checked via the FSM. I have no explanation for the shifting timing, but I would check those two items first.
 

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Discussion Starter · #14 ·
been a while, but found something very interesting -
was mucking around with the harness and looking to take it out for cleaning and organizing - my dad and I found that one of the CHTS wires was in the wrong slot on the connector that plugs into the ECU. That probably explains why the meter read 'no continuity' when trying to test the CHTS wires from the ECU to the pigtail per the FSM... we didn't even know we were testing the wrong wires. Got good continuity when testing the correct wires. When college work lightens up I'll go through the rest of the wires and see if there's anything else misplaced.
 

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Discussion Starter · #15 ·
Not too long ago I re-organized the wiring in my engine bay, and found that the power steering sensor connector was swapped with the CHTS connector (rather than wires being swapped on the ECU harness connector). Not entirely sure when they were first mixed up, maybe sometime around when the heads were going back on the engine after they were rebuilt. Lately though while running the car I've been getting an issue where after it's warmed up a bit the idle will become shaky and the rpms will decrease. I can tell when this is starting to happen as the engine starts getting thrown out of balance which causes the whole car to shake. When it's at its worst, from pulling wires off of the distributor at idle to see which cause any difference, I found that it was running on maybe only 2 cylinders! Pulling only the #1 or #5 wires would cause it to immediately stall, while all of the other wires would make no difference.

I've been told by someone in a z31 owners chat that these symptoms (runs good on cold start, bad when warm) are caused by a bad crank angle sensor. Out of curiosity, I tried swapping around the 2 spares I had lying around from junkyard Zs - none of them really worked. One of the ones I had looked like a newer design and it worked a bit better, but was still very finnicky when hot. The other older one didn't make much of a difference at all.
Can anyone confirm that this sounds like a CAS issue?
108753

The one to the right was the sensor that sucked the least.
Here's a video of the bad running condition -
 
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