91 Z 350 TPI falters when it gets warm. Ran well up to about two weeks ago. Initally tach started to wiggle then engine broke up and stalled. The tach wavering has tended me towards electrical but cannot rule out gas. This last time it did not restart until the engine cooled. Can hear fuel pump work. Thought problem was connector from pick up coil to module but only worked short time. New pick up coil installed, new cap and rotor. Tried numerous modules, new coil, pig tail wire from coil to module. New ecm did not help put old one back in same with coil. Module is Delco. Thinking it may be an open wire after engine heats, but from where?

 

I'm having similar symptoms with my 91 Z-28 w/ 350tpi and I've seen a few others with the same troubles; however, no one has a for sure answer on how to solve the problem. Mine acts up pretty much the same except when mine cuts out it'll fire right back up, no problem. Drives fine in the morning when it's still a little cool out; but when driving home in the afternoon heat it hesitates upon acceleration and cuts out at red lights. No sputtering or rough idle; just cuts out when I'm at a stop. The hesitation is intermittant; it'll do it one minute, then be fine another, then act funny again.

I've thrown plugs, wires, cap, rotor, coil, fuel filter, air filters, timing, idle air control valve, CPU reset, and gas treatment at it to no avail.

My next step is check the EGR valve and maybe the oxygen sensor. Might also take a look at the MAP sensor; might need a little carb cleaner spray. My last resort is the dreaded fuel pump change but I'm not to sure it'll fix the problem. I'm hoping it does so I don't have to pound my head against the wall.

The most suggested fix I've seen on these forums is to check the timing. I'm at a loss myself.

 

I say this in jest, but it is nice to hear that I am not alone this has been a real pain.

We did everything you mentioned you did.

My problem just disappeared after I cleaned a connector that was located near the AC/HT fan motor. It was covered with a tar sustance that could have insulated the wires. I had already made an apt with a real electrical expert so I went there with a motor running perfectly. He said he could find nothing wrong. He also said that the plug was associated with the AC and would not effect ignition but the car now works fine.

My tach would fluctuate before it stalled as if it was getting an erratic signal from the coil. I believe it is without a doubt an electrical issue.

Also clean the plug from the pick up coil to your module in the dizzy.

 

Hmm.. I have a similar prob, but the tach jumps and is only reading about half the actual RPMs. Car misses at idle and just off, but when you give it a bit of pedal it's great. It occured the day I did my cap/rotor/wires.. the distributor makes a ticking sound and a slight wine/whistle.. I changed the cap/rotor back, no help.

I've gone through most of the engine and emmsions crap. Only thing I havent done is pull the distribuotr and really test, the injectors or the computer..

But, the tach was fine before the issue... time to find wiring diagrams.

hmm.. I gotta try to fix this tommorrow..lol

 

You have an easier problem as the issue began after you did something. Keep checking everything associated with the cap and rotor and wire. Sometimes the new rotors are tight and do not slip into place completely. take some emmory cloth to the dizzy shaft that the rotor goes over for an easier more certain complete fit. Good quality caps are better, Accell and the like. Tach tells you that you are not getting all the engine spark pulses as in a coil prob..

 

Replaced the distributor; problem fixed!

Turns out that the distributor gear is made of brass and eventually wears out. At low RPM, some of the worn out teeth would not catch and cause the car to stall out. At high RPM, the distributor would rotate fast enough so that there was not a problem.

I also replaced the hose going from the PCV valve on the passenger side to the throttle body. It had a hole in it and upon acceleration, caused my hesitation.

 

has anyone tried replacing the Ignition control module?

it is just a thought but if i remember correctly the tach gets its signal from the ICM and they are prone to heat failures... and notorious for testing good when they are in fact toast

 

I just fixed a similar problem in buddy's 89 Firebird 2.8. Roughish idle and hesitation

Pulled the wire from cylinder 6 and put a matching plug on it and grounded the threads to the manifold and got NO spark. WE tried 2 more cylinders' wires and they were fine. A new wire from autozone and prob all fixed.

On my honda I had spark on all 4 but is a dim yellow. The car wouldn't even start. HondaCivicForum.com tech info stated to replace the coil if you were getting yellow orange spark. Replaced the coil and problem solved.

 

Mine was doing something similar and so was another guys IROC at the texas thirdgen gathering. My car stumbles, falls on its face, then diesels for a few seconds, sometimes dying. Then she comes back to life, with no other problems. It does this about once a month and yes, the HOT weather seems to be the culprit. I havent tried anything yet because it seems too vague to throw parts at but, it wouldnt hurt to do a "tune up" if it seems to be working for yall.

 

check your injectors guys...they should read around 16 ohms...
I would bet that some or all are less than 10 ohms...

 

Heat, heat, heat causes electrical deterioration. Distributor module, injectors, and far less likely to be suspected is what happened to me, (I replaced everything first, dist, coil, injectors(2 were bad)) is the (4) wires that plug into the module that goes to the computer. Engine heats, wire resistance builds up, screws up signal going and comming from computer and bingo, car stumbles and misfires. Solution- cut wires at plug and run external set to computer and solder. It does not pay to disturb the big wire bundle trying to find the four wires and removing them. You can put a fuse in the one wire that controls the computer cutout for timing adjustment. (pull the fuse when you time the car) You can use a plug or whatever also. Nothing better then knowing you have a strong signal going an coming from the computer.

 

Ancient thread, but I figure it's worth adding my experience to. I had this issue with my '89 5.7 tpi Iroc-Z. The longer I drove the car, the more it just felt like it would randomly cut out on the freeway. Once it got warmed up enough to do this, it took about 30 minutes driving, it would do it worse and worse the more you drove it, to the point that after an hour or so, only light acceleration was possible or it would stumble and stutter like mad, all without a check engine light. Turning it off and on again made no difference.

My first step was to change the cap and rotor, since I didn't know their age and the plugs/wires were recently done. When that didn't work, I gave it a lot of thought and swapped the ignition coil. It was a bit annoying, the old one looked original to the car and was riveted to the bracket, and I don't have a cutting wheel. But after the swap, the car not only seems not to have even a trace of a stumble when warmed up, but it also seems to have a lot more power. I'm thinking that the coil was acting up even when cold and it was producing random multiple misfires that weren't consistent enough to feel or hear otherwise and just had to get warmed up to produce the big stumbles. I know from experience the car drives on 7 cylinders without roughness or weird engine note.

Edit; It had this issue for quite a while, slowly getting worse, and I always thought it was the fuel pump dying, so I never looked into it further. After the pump did fully die, I had the tank and pump swapped for Aeromotive units and was surprised to see the stumble reappear as it was before.