mcge8241

I’ve been trying to solve a strange issue with my ’84 and am hoping someone on here can point me in the right direction.

The car is an anniversary edition, manual trans, turbo. I have a big K&N filter on the stock intake pipe, have removed the cat, and have a high-flow muffler on. Timing is good and I put on new plugs and wires.

Essentially, the car runs great when it’s warmed up. No hesitation, no stumble, nothing like that. It idles smooth and revs smooth. When the car is cold, it actually starts pretty easily and idles fine most of the time. The issue comes when you try to accelerate it. Most of the time, it will refuse to rev. If I floor it, the revs will drop below idle and it will burble and cough for a long, long time (10-15 seconds) and it will finally rev up very quickly. But, even at that point if I try to keep the revs high and maintain a throttle position somewhere between closed and WOT, it the revs just drop right back down. If I let it idle till it’s up to temp, or even to where the temp gauge on the dash shows something (like 120?) it starts to rev smooth and eventually runs just fine.

So my thought was immediately that the CHTS needed to be replaced because the problem seems to be entirely engine-temp dependent. I didn’t check the codes at that point (didn’t know where the ECU was located). Well, I dug into the front of the engine to replace it, and much to my surprise it looked brand new. I couldn’t get the thing out without breaking it, so I simply decided to run a few tests. I first checked the resistance as indicated in the manual (now I knew where the ECU was). All of that seemed to be in good shape. So I also checked the new sensor (plugged in but not installed into the engine) and that too seemed to read OK. Next, I tried to start the engine with the new sensor plugged into the harness but not installed into the engine, thinking that cold would be cold regardless of where the thing was put. No change at all—still had the same issue. So, not wanting to mess with that sensor anymore, I re-installed everything and moved on.

Next I looked around online and decided to try the MAF next. I tested my MAF by checking the resistances. There were no dead spots, but the resistance was way different than what I had found online. Ordered a new one and installed it, still no change at all.

Next I tried checking the engine codes. I disconnected the battery and let it sit, then plugged it in and went through the whole “let’s try to make it rev” thing. Shut the car off and read the codes. I keep getting the following:

14 – Vehicle speed sensor (this is weird because sometimes it works and sometimes it doesn’t. I’ve tried two different sensors and both have the same issue…it’ll work just fine and then it’ll stop, then it’ll work again randomly when driving).
23 – TPS sensor (this is weird too because it seems to operate just fine when I test it with my multimeter…closed when throttle is closed, etc. I’ve heard this might be an issue with how I am checking the codes actually…that there is a procedure to press the pedal at some point to avoid having this code? Have ordered a new one and harness just to be safe, but they have not arrived yet).
24 – Neutral safety switch (this one I know nothing about…not sure where it even is)
31 – AC/load (again, I don’t know anything about this one…I’ve checked the connections to the thing on the intake manifold and they seem to be ok…)

So now I’m stumped. It sure seems to be something engine temp dependent. I’ve thought about ordering a fuel pressure regulator next, but I still get the feeling it’s sensor related.

I’ve searched high and low on this forum and on others, and it seems like a LOT of people have had this exact issue, but no one ever seems to report back with what (if anything) was able to fix it.

Any help would be greatly appreciated!

rogerz

try unplugging the O2 sensor. driver side low down on exhaust pipe. NOT your tps. NOT safety switch and NOT AC. Chts and O2 both temp related. Check the wiring to the CHTS. WAS the connector in good shape. Also look at the oval connector atop the engine. I usually cut all that off solder in some good wire and a connector. connector are just a point of failure and are only there for convenience in manufacture. chances of having to replace chts more than once are low. Have you checked your fuel pressure? How is your vacuum? vac leaks the bane of FI engines.

all your symptoms point to the chts in my experience. twere me I'd replace the rascal anyway. there is a post on line somewhere about replacing it without taking off the tin but you really don't have to take that much off to access it. the wiring to it is in a high temp area and tends to get burnt. here is some general stuff to do that may save you a heartache in the future.

Get your battery load tested. Be sure you have clean, tight corrosion free terminals on both ends of your battery cables. Be sure the ground (negative) cable goes to a bolt into the frame before going to the starter. grounding through the starter is not a reliable connection. Z's don't like low voltage. causes the electronics to act funny. it is possible to have enough amps to crank but not enough voltage to run the electronics. If you have one size fit all cheapo clamp on terminals they are a problem waiting to happen.

Clean the connectors for the maf or afm, ecu and tps. Deoxit by CAIG is probably the best connector cleaner on the market. spray with CorrosionX after cleaning and before putting together this will help prevent any further corrosion.

mcge8241

YOU, good sir, are my HERO!

Just went out to try unplugging it and noticed an immediate difference. She still stumbles slightly, but revs up way, way, way easier than with it plugged in. I did a sanity-check and re-plugged it in, tried it a few times, and then un-plugged it again and tried it again. Absolutely a big part of the problem I'm having.

Thank you SO much for posting! Much appreciated. And here I was --this-- close to just swapping the thing over to megasquirt.

duowing

There's a couple of things. This sounds like it could be a CHTS or an O2 sensor problem. I'd drive for a while without disconnecting the battery and after several startups check and see if the CHTS code comes back. If you can try replacing the O2. A lot of cases when the O2 goes bad it will stick in either a rich or lean mode which seems to then just keep throwing that signal at the ECU. So the ECU is constantly seeing way too rich or way too lean. I've noticed almost no difference running a Z31 ECU with or without the O2. I'm also talking a good functioning O2. One thing I have encountered is when I converted my 280ZXT to the Z31 ECU/MAF combo I used an 84' ECU. Right at first when cold it would be alright, and it would be for a short period which would be around the transitioning cold to warm phase, the car would stumble, pop, etc. Then it would get past that and be fine. From everything I've come across this is literally a problem with the early 84'-85' year ECUs. Something about with the way it uses the cold/warmup values into the closed loop or warm mode. When you turn your car ON, but don't start it you should hear your fuel pump run for a couple seconds, then shut off. Does it do that or does it continue to run? If your fuel pump seems to continue to run indefinitely this could be a sign of your ECU going goofy.

daveg56313

After several years fighting this problem on my 84 Turbo and trying all of the suggestions here and on other websites, we found the source of the problem was where the exhaust pipe bolts to the exhaust manifold. Two out of the three bolts were missing and the pipe was loose so the pipe was open enough that the O2 sensor readings were thrown off. After replacing the bolts and closing up the gap in the exhaust manifold to exhaust pipe the stumbling went away! But then it failed the emission test and we had to try to get the O2 sensor out. One broken wrench and several attempts with a torch and penetrating oil later, we got the bung to unscrew. Replaced it and the O2 sensor. Now it passes emissions and still no stumble!