1990 Chevy 4x4 low speed miss

[Free88]

What I meant was that there is no check engine light on. I did not realize that it would store codes without some kind of warning light on the dash. I will look for the post on how to test for codes without a code reader. Sorry if I confused things.

[ChevyTech]

Here are some web sites with the basics on retrieving codes without a scan tool or code reader.

Poor drawing of ALDL but ok instruction + codes
http://www.cfm-tech.com/gm_tbi_trouble_codes.htm

OBDI terminal identification
http://robertpowersmotorsports.frees.../ALDL_plug.jpg

OBDI illustration and instructions
http://www.troublecodes.net/GM/

1985 – 1990 trouble codes instructions
http://www.iroczone.com/specs/TroubleCodes.htm

[Free88]

Thanks. I will try this as soon as I get a chance today. Right now I am going to block off the egr valve and take it for a run. If that does not work I am going to swap ecm's ( computer? ) and try that. Are they different from a 305 to a 350?

[ChevyTech]

Look at the ECM numbers.
The same ECMs where used for both engines for a number of years but the chips (Prom) would be different, but look at the numbers on the ECM. Depending on the vehicle and equipment some vehicles used a different ECM.

[Free88]

Thanks for all the help. I think we are closing in on the problem. Due to all the work on the 91 transmission today I was not able to do much with my truck. Wifes 91 always comes first. Happy wife, good life. So I disconnected the egr valve today and this eliminated a lot of the problem. Still a little rough but not as bad. I also noticed a hole in the y pipe today just close to the oxygen sensor. Wondering if that may have anything to do with it. Anyways I am going to the city tomorrow and will be back to the truck either Sunday or Monday. I will let you know how it is after I remove the EGR and clean the valve and port. Have a good weekend and enjoy your time with family. Thanks.

[ChevyTech]

You're welcome.

So I disconnected the egr valve today and this eliminated a lot of the problem.

Note: A lean condition caused by another problem can be magnified when the EGR valve opens.

EGR valves have a spring in them that can get weak, or break, causing the valve to open too much or to easily.

I would suggest only using a general motors replacement EGR valve, if you decide to put a new valve on it.

If the fuel pressure is low, this will exaggerate the sag in power when the EGR opens.

I also noticed a hole in the y pipe today just close to the oxygen sensor. Wondering if that may have anything to do with it.

Yes!
Fix the leak.
Air getting in the exhaust system can also affect the o2 reading. In open loop operation (cold) the ECM ignores the o2 sensor reading, but in closed loop (warm) the ECM uses the o2 reading to determine if the engine is running lean or rich. If the exhaust system leaks ahead of the o2 sensor, (or just behind it), the pulses in the exhaust will draw in air and cause the o2 sensor to get an incorrect reading. A vehicle with an A.I.R. will have the same problem if the air management valve allows air to be injected in the exhaust manifolds during closed loop operation.