Combustion problems with a 1986 Chevy 350/5.7 engine

mcshamusSeptember 5, 2010

It's in a 1986 Chevy G20 Van, V8, Calif. It's a 3/4 ton, the VIN says it is, or was, a 5.0 liter V8 4BBL LF3. It had a 350/5.7 factory engine installed by a local dealer in 1998. There's about 125,000 miles on that engine.

I've had this van for about six years. I've never driven it much, it always started, but it needed an alternator. I recently put in a new battery/alternator and rewired the starter. After that it had acceleration and combustion problems. It runs and sounds great at idle, but under load, at low speeds, it hesitates, sputters, stalls and occasionally backfires. Some of the symptoms indicate a possible problem with the computer controlled units on the carburetor, the Mixture Control Solenoid or the Throttle Position Sensor. I started looking through the wiring and fuses and found out how to read the ECM/MIL flashes on the Check Engine Light. The first time I checked, there was trouble code 34 - indicating a problem with the MAP sensor - but, since then, no other codes have showed up. Now I'm looking into the 22 pin and 18 pin connectors on the computer module, tracing wires, testing and looking for trouble. I have a volt/ohm meter, a cheap tach, and a good hand held vacuum pump.

The truck has a rebuilt, Rochester Quadrajet E4ME - 17085524 - carburetor.

The ECM is labeled, re-manufactured, 01226865.

The PROM chip is marked, DELCO CE1090N 8520.

What I need is Help in locating and identifying all those sensors, vacuum switches, valves, relays, and threaded plugs, screwed into the engine in various locations. I could use Links to web sites with information on how to do step-by-step testing procedures for troubleshooting and diagnosing the problem.

Tips from your personal experience, clips from your private archives or just plain sympathy will be welcome.



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A bad MAP sensor can cause the symptoms you describe.

Also, check all vacuum lines for breaks/leaks. Insure that all are properly conected. Some of the vaccuum connectors may be ported (Have internal restrictors). If any of these have been replaced with regular connectors, system upsets can occur. A very good repair manual is required to identify which connectors, if any, are ported.

Back in 1986, the ignition system may have used a conventional distributor enhanced with some electronics. (Be gentle with me here. I'm flying blind relying on memory alone.) The ignition advance was still by governor springs and weights plus a vaccuum advance unit. Engine power can be reduced at part throttle if the vaccuum advance is missing - also the vaccuum will be down because the throttle is opened more that normal. Abnormally low vaccuum will cause the enrichment needle to rise in the carbureator and excessively enrichen the fuel mixture.

    Bookmark   September 7, 2010 at 6:10PM
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The whole thing is kinda "Ported". Hard plastic tubing, about 1/8 ID goes to the MAP sensor, 2 vacuum diaphragms on the carburetor throttles, into the air cleaner and back to the intake manifold.

It's got a HEI distributor system with NO vacuum advance canister, centrifugal advance ONLY. It's Calif/Emissions and it's a poor system for setting a good timing curve. I found out.

I looked at the timing w/ my inductive light, it's about 10 degrees advanced. But the timing degree tab was missing.

Pulled the plugs and found TDC for no.1 cylinder.(I broke the insulators on 5 plugs because the 5/8 socket I had was not deep enough for the plug and ratchet head at the same time and slipped).

I still got the same problems.
I'm looking for error codes on the check engine light.

Thanks for your contribution,

    Bookmark   September 20, 2010 at 3:16AM
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