OK 88 chevy truck 3/4 ton
350 TBI

Engine is hard to start without opening the throttle slightly, when it starts I let the throttle close, it sputters a bit then smoothes out but it idles extremly high (2500-3500rpm) When I give it gas it will do 1 of 2 things, spit and sputter or just die. I first pulled trouble codes and got a code for EGR and a code saying there was no signal from the knock sensor. I suspected a bad vacuum leak. Here is what has been done so far

replaced the following parts with new ones: EGR valve and gasket, Knock Sensor, IAC, Throttle Body Gasket, plugs, plug wires, cap , rotor, installed MSD Pro-Billet Dist with new module, coil, all vacuum lines have been replaced.

Still have the same problem So I checked the intake gaskets for leaks, seem to be OK.
Now I suspected a Burnt/broken valve or seat
Did a compression test all cylnders are good so that rules out valves or head gaskets or rings

All spark plugs look normal after running a while

It really acts like it is a massive vacuum leak, but where?
How likely is it that the intake gaskets could be leaking on the underside in the lifter valley?

I have also tested my MAP and TPS, both are good

Is it possible a bad fuel pump would make an engine idle high?
I know the throttle blades are completely closed so It has to be getting air from somewhere.

Could my ECM or ESC be the culprit? If so how do I test them?

Every sensor on the motor is new or have been tested so the only other guesses I have are:
Intake leaking on lifter valley side.
Bad fuel pump?
ECM.
ESC.
Has anyone else had a similar problem?
Does anyone else have any suggestions.
Thanks
Jimmy

 

try checking your timing chain, if it is off just a couple of teeth, the vehicle will not run right. Had the same problem with a dodge i used to own.

 

Jim,

Yes, I've had a very similar problem that torked me for about a week. (See #2 below) A failing fuel pump shouldn't make the idle that much higher, but would certainly affect mixture once you're in closed-loop mode.

Possible vacuum leaks are:

1. Power brake booster (unless of course your's is hydraulic assist from the P/S pump);

2. PCV valve problem (I had the same issues on an '88 Astro 262 - turned out to be a bad brand new Fram PCV);

3. Intake gaskets, which would likely show up with a vacuum gauge;

4. Incorrect EGR valve type staying open, or stucj EGR solenoid;

5. HVAC control vacuum lines (althought they are very small and wouldn't constitute a "huge" leak;

6. EVAP purge canister valve sticking open;

7. Leak in the AIR control valve.


I would suggest removing and capping all vacuum lines one at a time, noting the changes as you do. One of them may solve your problem.

 

¿Any updates?

Valve timing is correct, vacuum is low, compression is a little uneven but not low enough to indicate a castastrophic problem. However, "blowing off the rocker cover" is a sign of excessive fuel in the sump, at the least. First, check the oil for fuel and change it if necessary. Then find the reason for the excess fuel.

 

Vader,
did you get my updates?
some one else suggested a bad CTS could cause this prob, what do you think?

 

A completely failed CTS would set a DTC - 15 I believe. An incorrect CTS can skew the mixture and cause a higher idle, but the target idle is still only a maximum of about 1,500 RPM for a cold engine.

You may want to take a close look at the IAC motor and pintle to make sure they are not damaged. An easy way to test is to pack soem modelling clay in the IAC air passage at the rear corner of the TB. This will force all idle air through the throttle bores of other sources.

It may also be time to get your hands on a data-logging scanner to look at IAC counts, target RPM, sensor inputs, and fuel pulses. You've covered most of the easy things that I can recall.

 

Any suggetions on where the fuel may be coming from, a bad FPR? What do you think is causeing the backfires all the sudden, could I have my Dist. 180 off? Do you think this bacfire/explosion could have caused any internal damage?
Jim

 

What kind off scanner do you suggest? Will it give me any more info on OBD-1 vehicles than just grounding the ECM.
Could my problems be the act of a bad ECM?