95 Suburban Stumble/backfire ECM Updates?

[BSarte]

I bought my 1995 Suburban in Dec. of 2000 and love it. It ran great until about 2 years ago and the problem I am having in short is a stumble/backfire after it warms up, but is not consistent.

I have diagnosed (have a PC program to hook up my laptop, DIACOM) the hell out of it, put in alot of parts/sensors as well as money, and the problem still persists.

It started about 2 years ago with a slight stumble and miss under acceleration. I have had it to 3 different dealerships, a supposed "GM" guru, and this morning I spent 4 1/2 hours with the guy who I trust the most, my old mechanic.

I have replaced the injectors, O2 sensor, TPS, Distributor module, Catalytic converter, MAP sensor, EGR, ECM temp sensor, knock sensor, plugs/wires/dist cap & rotor, as well as to swap out the ECM and Memcal with known good ones. Problems still persists.
This morning my "old mechanic" and I came to the conclusion after trying many different tests (fuel pressure, SNAP-ON Diagnosis analyzer, timing, etc...) that it appears to be running lean according to the O2 sensor and why the ECM/injectors are not compensating correctly is now the mystery. My mechanic wants to keep the truck for a few days to further diagnosis the problem. I would like to try to help him since we are both no dummies, but are quite stumped by this one.

Just before this problem started, the transmission was replaced under warranty by the local Chevy dealer and one of the items they noted they did was to "update" the ECM. I recently went back to them and asked about the update, but they seem to now have no record of it. I personally think that my problem is with either a wiring harness issue (wires shorting) or with something in the ECM such as the "update" that was done. Anyone have any ideas as to how an ECM/Memcal update is done?

Is it like updating the bios on the PC?

Did I get a bad update?

Your help is appreciated.

Bruce

[Fedexjetmx]

Hello
How does it idle when warm..normal.Is the stumble off idle as in accelerating from a light, and or does it do it at steady speeds. When it does, does it do it only once or continue doing it until shut off and restarted. How many miles on it? The reasons for all the questions is. I too have a 95 4wd Suburban w/5.7. Although I did not have you exacte symptoms, mine would slightly stumble on off idle accelration, the heavier the acceleration the less noticable it was. I was a little perplexed because I am a mechanic although not in automotive in almost 20yrs, I'm an aircraft mech..back to subject. I have all the goodies a dealership has because I fix alot of cars for other people plus my fatheinlaw has a trans shop. My problem boiled down to where I was buying my gas. I'm a cheapskate, lets face it most people will buy gas a the cheapest place. Try changing your fuel filter use some fuel treatment that disperses water. Just a sugestion not that your problem is the same as mine......but sometimes it's hard to see the forest for the trees. If you know what I mean.

[pdalton]

I posted something longer about this elsewhere, but I had a similar problem with my 97 that the dealer couldn't get correctly diagnosed with the computer, but said it might be failure of the Mass Airflow Sensor, Intake Air Control Valve or Throttle Position switch.

I bought one of each from an independent parts house and installed them one at a time in order of increasing cost, testing after each. It rurned out to be the most expensive -- the Mass Airflow Sensor, which also was the easiest to replace.

Don't know if this is any help for you, though.

- Paul

[Delta Dart]

"I personally think that my problem is with either a wiring harness issue (wires shorting)"

You might be right because as you stated "Just before this problem started, the transmission was replaced ". There are many opportunities to damage wires during a tranny remove and replace. I hope the mechs working on your Sub have taken this into consideration.

Sorry no info on the ECM update.

[amac209]

i'd be looking for a vacuum leak check all the intake gaskets and lines. do you have air injection? sometimes the diverter valve blows out and causes similar problems. did you take it for a drive with the fuel pressure gage hooked up??? the best way is to hook up an extension and tape the gage on the windshield, sometimes pressure is good at idle but doesn't increase with acceleration. a vacuum gage is another good diagnostic tool for finding sticking valves or other problem. replacing the fuel filter is a good start if you havn't done so already try to blow through the old one. check the egr controller too sometimes they leak or get cracked. i asume you reset the computer after all the sensors were replaced this needs to be done so the computer can recalibrate. hope this helps.

[toivo]

try wiping off the wire in the map sensor. if it gets dirty it does not work right. be very careful though, it's a small wire.

[toivo]

try wiping off the wire in the map sensor. if it gets dirty it does not work right. be very careful though, it's a small wire. whay about the pickuo coil?

[jfmctlaw]

Wow. I have a 95 5.7 K 1500 w/193K miles. I just had the tranny replaced with a Chevy reman by an independent shop. The missing/skip/misfires started a few weeks later and I have replace my fuel filter, O2 sensor and Cat. I'll do the plugs, wires, cap tomorrow, but now I'm wondering about this tranny replacement deal.

[dirty dan]

I've not had any experience with it, but I've heard that GM had wear problems on the distributor gear for the '95 models. It might be something to check.