RPM hunting with no load.

[ddan]

I have a high miles '88 3.0L 5-speed that has excellent oil pressure and mpg, but recently lost its smooth running, easy starting character.

The problem started gradually with harder starts which seemed to get worse after I replaced the passenger side valve cover gasket. Now with cooler weather (I live in snow country) it idles like a dragster, but the exhaust looks clean and doesn't stink. It is not a daily driver. It sometimes sits for a month at a time, but it has a nearly new battery that really cranks!

Since the surging rpm only happens while sitting in neutral, I thought the bypass valve was a liklely culprit or a vacuum leak from one of the lines disconnected to remove the valve cover.

I checked and resealed lines, wiggled some others with the engine running, to no effect. I also checked the PCV valve because the cover gasket was replaced due to oil leak. Removing the valve didn't affect the engine and it had plently of suction.

I removed the bypass valve and cleaned it with carb cleaner and a Q-tip. I then tested the valve solenoid with battery voltage and reinstalled it--no change. I wedged a tee fitting into the throttle linkage to raise the idle so my hands would be free to pull the power to the valve. The rpms decreased, but remained just as unstable as they were under bypass control.

While checking the vacuum connections at the manifold tree I was shocked by a bad plug wire. I replaced it and checked that plug. It was clean with a gap of .047. With the plug wire replaced I can now put my bare hands all over the distributor with no sensation. Replacing the plug wire didn't stabilize the rpm.

The fuel pressure is rock steady at 42 psi no matter how I jerk the throttle or how the idle surges. The starting psi is less than 10, but with restarts the pressure climbs above 42 psi. The fuel pump is a turbine style instead of the original positive displacement style. The little door inside the fuel filler no longer closes.

When I disconnect the passenger side injectors the engine very slightly stumbles and recovers. Reconnecting the injectors has little or no effect. They are far enough upstream to share? The driver side injectors are inaccessible with the engine running. I dumped a pint of Gunk injector cleaner into an almost empty tank , added 1/2 gallon of gas and drove it up to freeway speed briefly. I let it idle a while, then let it sit overnight. Added another 1.5 gallons of fresh gas and it still ran like crap.

Removing the air cleaner does nothing.

The engine runs quite smooth at speed, but once or twice on a freeway trip I will feel a "headwind". It immediately recovers so I don't even move the gas pedal. I haven't gone on a trip since replacing the plug wire so maybe that quirk is fixed.

While sitting in neutral I'll try to hold speed around 1500 rpm and it sometimes surges rhythmically. Other times it will hold fairly constant for a minute or so, then surge to 2000 or more and hold. It sometimes seems like the computer isn't getting a good rpm signal. I'm guessing it derives rpm from the distributor?

It's in the 40s now. I wish I could retest in 70 degree weather!

P. S. I also swapped the throttle position sensor with an old one I had lying around. It ran the same so I swapped back.

 

[BrooklynBay]

Welcome to this forum! Did you check the vacuum line going to the MAP sensor? Are there any codes with a check engine light? Don't use carburetor cleaner on a fuel injected engine. Only use sprays which say that they are oxygen sensor safe. Lubricate the idle air control valve (also known as the air bypass valve) with WD-40. Clean the air charge temperature sensor, and the O2 sensor with intake cleaner.

 

[Josh P]

Is the issue the same when the engine is cold as well as warmed up? The engine coolant temp sensor controls the idle and fuel on cold starts, as the sensor warms up, the idle will drop. When the sensor fails, the ecm doesn't know the engine has warmed up.