This one has me pulling my hair out. 90 6cyl pickup. I bought this truck a while back and at the time it ran decent until I put a wrench on it. It has never been the same since. I started with a tune up and a few minutes into the test drive, it started to stumble. I found the injector wires pinched under the air cleaner so I soldered on two new injector connectors. Truck ran good for a day. Then the off idle stumble came back. Swapped ecm's, same thing. New distributor, new or replaced with working map, knock sensor, tps, iac, egr, esc module. Had injectors cleaned and flow tested. New fuel filter. Same thing. Engine bumps about every second starting just off idle however full throttle is slightly better but anything between just off idle and full is totally undriveable.
I replaced all ecm ground straps and also added new to bring all grounding points back to battery. Truck ran like new for 1 hour then back to the same stumble which I am 99 percent sure is the injectors not firing properly. Fuel pressure is good even when stumbling. Truck starts and idles perfect. Throttle response in neutral is fine but under load, it just starts choking. I can't find any vacuum leaks and the problem definitely appears to be electrical. I just ordered another fleabay ecm in case I fried both of mine with the injector circuit short.
What am I missing here guys? I've literally been screwing around with this truck for 6 months and I am completely out of ideas. If I actually needed this truck, I'd be in trouble. I thought for sure that I had it licked with the new grounds but then it came right back. Did I kill both ecms with the injector short? I usually love tbi trucks since they are so simple to fix but this one has been one for the books. I'm ready to give up if the next ecm doesn't work. Any ideas would be greatly appreciated.

 

Does spaying the ignition wires do anything?
Might have an ignition coil going bad.
Hows the cap & rotor? Any carbon tracking?
Use wire serperators on the ignition wires, sometimes crossfiring can occur.
Did you ohm the injectors?

 

Ignition is 100 percent new, no arching, cap looks good (switched just to be sure) new coil, distributor, module, wires and plugs. All wires are separated.
Injectors both measure 1.3 ohm. Had them professionally cleaned and flowed and they were right on factory spec after cleaning.
I am positive that the problem lies somewhere in the injector circuit seeing the changes that I saw with fixing the injector wires and regrounding the ecm. I just don't know why the problem disappeared and then shortly returned.

 

Check your alternator - it maybe putting AC ripple on the DC main bus once its gets hot. In CS130 alternator uses internal regulator and when it is on its last legs it may causing problems like you are describing. To verify this you need to have oscilloscope or take the alternator to electrical re-builder that can check.

Another area to check is distributor harness connector - make sure that you do not have broken or frayed wires - connector pins.

//RF

 

Reading your post you say it ran good for about an hour after you made a bunch of repairs. One being the ESC which a lot of people confuse with EST/ICM in distributor. Although it may happen I have never seen an ESC go bad... it takes the signal from Knock sensor and changes timing to EST.

Now if you replaced the EST in distributor and did not use heat disapating paste, or if screwss were not good and clean as they are the ground for that module, then it may go bad in an hour. It is the only part of the TBI system that I and many have seen be glitchy. All other sensores etc... just work or fail. Stock GM EST are known to be more robust than auto Part store replacements.

Can watch timing with light and see if it is glitching.

Since you had a wiring issue and you say you have a load issue I would check wiring to MAP.

Does the vehicle set any error codes?

HTH!

 

Now the alternator ripple is something that I have not thought of. I will definitely check that out. That "type" of problem is more along the lines of the screwy stuff I'm seeing. Great idea.

I did actually swap out the esc, but no change so I swapped back to original. I too have never had one fail, but I'm willing to try anything at this point. I also changed the ignition module several times and I only use real heat sink paste, not the silicone grease that comes with them. Spent more time looking at a timing light than I care to think about, ignition is solid. And no codes, at all, ever. That is what is really aggravating. I've never had a vehicle run this "weird" and never set a code. I've done a few runs on winaldl in the past, and all sensors are responding. This thing just chases it's tail. Data is all over the place until you back off and then everything comes back into line. Real frustrating.

 

Disconnected the alternator today and took it for a drive on battery only. No change. I was really hoping that the alternator had a bad diode in it but no luck. Back to the drawing board.
We'll see what swapping the ecm does when it gets here. Now I'm hoping that I've cooked both of my ecm's (bet you don't hear people say that very often) and that third times the charm now that I've got good injector wiring and perfect ecm grounds. But I'm not real optimistic at this point.
Keep the ideas coming. I've obviously overlooked something, but what?

 

Check these solder joints. The wire that GM used is nickel based and is difficult to solder. There is a good possibility that the joints cracked. Causes all sorts of strange behavior.

I don't recall GM using solder on the copper wire to injector wire connections. IIRC they were only crimped. Or, the harness ran that special wire all the way from the ECM to the injectors. I've seen both methods used.

RBob.

 

I will definitely recheck the solder connections although, they soldered up real easy. I have had some real hard to solder wires in the past but these soldered like butter. I've even gone so far as to feed the injectors power off a relay to eliminate any possible voltage drop on the hot side. The best I've had the truck running was with the injectors fed from the relay and immediately after redoing all the truck grounds. It ran like a top for an hour, I parked it, and the next day it was right back to bucking. The only thing that has really changed in the symptoms is that in the beginning it would sneeze through the tb every few seconds like the injectors just stopped firing momentarily, and now after re-grounding everything, it has a very quick and consistent miss while accelerating.

 

Alright, swapped ecm's today. No change. I did use prom from old ecm but I don't think that would matter unless someone thinks that this problem could be in the prom (I have tried two different proms though, both with same symptom.

Cut out solder joints on injector connectors and crimped, no change.

Is it possible that the ignition module might have been damaged due to bad ecm grounds? Should I try another one, again?

Once again, the only two times that this truck ran right were (1) when I replaced the injector connectors and that was running decent but short lived and (2) when I replaced all the engine grounds and that produced a perfect running engine and then it was right back to missing again.

This thing is getting closer to the junkyard every day. My wife is begging me to junk it and I'm starting to agree with her. I've just got too much into it at this point and it's really becoming a matter of pride. At this point, it's got me whooped and I'm getting tired of fooling with it. I've never had a tbi beat me before but this one may just become the one that got away unless someone can come up with something else. It just kills me to think of junking it after seeing how well it ran when the gremlins went to lunch (didn't take them long to come back though).

 

I would not be so fast junking it just yet. And yes, as matter of pride I would work through this mystery problem. So you'll have to tell your wife to slow down on the drum beat. BTW, flowers and candles usually come wife down for a few days. I guess this board is becoming a support group for TBI heads!

In my past experience I have seen brand new (non AC-Delco) replacement parts bad straight out the box. This included O2 sensors, ICM, coils, MAP & TPS sensors.

The cut out effect is very strange to say the least (I am involved in another mystery stumble) - but I would pull, check and replace EFI fuses (IRC 10, 20AMP???). When you pinched injector wires you may have stressed EFI fuse, but not enough to fully open - just enough to partially melt fuse material. As engine runs - current being pulled by injector coil fuse warms up and opens up causing a stumble, as it cools fuse closes. If you have oscilloscope you'll see +12 volt side of injector circuit drop out momentarily and after a while stay open. Put a DVM on +12 side while engine runs to see if you loose +12 supply when it stumbles and dies.

Recommendation REPLACE EFI FUSES (cheap and easy)


//RF

 

Did you check fuel pressure? Might have to do a volume check.
Take the cap off of the distributor, make sure theres no slop there. How does the pick up coil look? Is it rusty?

 

It is possible for any part to go bad! he Ignition module (EST) is one known for still running when bad. I have never found a GM diagnostic test for it. Parts stores have equiptment to test but unless you can talk the guy into doing it at least three times it's a worthless test, they usually fail test 3rd time up.

Since the only time it ran good was before a tuneup, look at all parts you replaced and anything moved. Since it ran good after you played with grounds and injector wires look at all the harness, grounds etc... sounds like a wire breaking?

Do you have a scantool or can record data? If so this uausally shows a glitch when it happens.

Both times you said it ran good for a little bit, was the engine cold?

 

RFmaster
As far as the efi fuse goes, I thought about that and as of right now, injectors are being powered by a relay that I wired up with the stock injector power as the trigger and actual power fused directly from the battery to eliminate any possible low voltage condition. I'll change the fuse anyway just to eliminate any possibility of it screwing with the relay. Also engine never dies, just stumbles.

ex-x-fire
Yes, I did a pressure test and even drove the thing hanging out the door holding the pressure gauge connected to fuel filter fitting to see if it dropped while it was stumbling (good thing all my neighbors are rednecks too or that may have looked funny). Pressure never dropped a bit even at full throttle. It sat right at 12 lbs if I remember correctly. Rebuilt tbi when I had injectors tested so it also has a new regulator.
Distributor is brand new as well as all ignition components so no slop, new cap, wires, coil, plugs. No change when complete ignition was replaced.

EagleMark
I've changed est several times but not recently or since I did get it to run good briefly. I've not been able to find any bad wiring but that's not to say that there isn't, I just cannot find it. Yes, I can log with winaldl and I probably need to do that again. Maybe you guys can look at the log and see something that I'm missing. When it did run good, it ran good in open and closed loop. Now that it's stumbling again, it will do so hot or cold although it does seem to get worse the hotter the engine gets. Not just closed loop but how long it has been running.

 

Alright, I finally got a chance to log the piece of crap today. If any of you guys get a chance to look at the logs, I would much appreciate it. I'm not that great at figuring out the data.

I started logging from a cold start and took it to town sputtering the whole way (wasn't quite sure I would make it back) and then logged again on the way home after a quick stop in town. Maybe someone can help me figure this thing out from looking at the aldl log.

I was getting a bunch of rich flags, just about any time I got on the throttle, while at the same time getting high blm numbers. I'm as confused as my truck. What could be causing this? Once again, when I redid the head to firewall to frame grounds, it ran like a top but only for a short while and then right back to the same old sputter.