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I am trying to diagnose a surging idle and stall condition on a 1990 GMC S15 Jimmy with 4.3 TBI.
The car initially presented with no spark. Ignition control module was replaced and the car fired up again. However, after this repair it has a surging idle, which it never had before the new module.
The surging happens whether the engine is cold or warm. When the engine is started, RPMs will raise for a few seconds, settle to normal idle speed, and a couple seconds later start dipping regularly every couple of seconds. After a minute or so of this the engine stalls.
A few things will keep the engine running:
1. Disconnecting a vacuum hose.
2. Spraying starter fluid into the intake when the engine speed begins to drop.
3. Disconnecting MAP sensor connector.
Throttle response is sometimes poor, but this clears up when MAP is disconnected.
No fault codes are stored.
Fuel pressure has been tested (tapped into fuel line upstream of fuel filter) and holds steady 13-14 psi at all times.
MAP tested, shows smooth voltage shifts in response to varying vacuum.
TPS tested, shows smooth voltage shifts in response to varying position.
All vacuum hoses have been sequentially disconnected and ports capped. No discernible change in idle when spraying starting fluid around the intake manifold. I have not yet smoke tested the intake.
IAC relearn procedure has been performed. IAC removed and cleaned, pintle verified to extend with key on and diagnostic connector jumped.
It may be worth noting that a cylinder contribution test once showed two cylinders not contributing. The engine does feel like it has a single or double cylinder misfire. Firing order was double checked and the two cylinders’ wires were swapped just to be sure, then swapped back again, with no change. About an hour later, the contribution test was performed again and all cylinders were found to be contributing.
