CHEVY TBI AND /OR DIS ISSUES

1995 Chevrolet Truck

Tiny

kalanikaau

June, 5, 2009 AT 5:43 AM

Engine Performance problem
1995 Chevy Truck V8 Two Wheel Drive Manual 150K miles

Bone stock code H (305) engine mated to a 5 spd. Manual trans.
About 6 months ago, during a severe thunderstorm here, the truck suddenly and un-expectedly starting running badly, no power off of idle, increasing throttle opening further would cause it to stall.
This truck is used infrequently so I just ignored the problem.
Now the problem has progressed to the point where I need to manually prime the TBI bores with carb. Cleaner just to start it Once it does run, I need to feather the throttle just to keep it running. Also, a huge amount of air bypasses the IAC, creating a massive vacumn leak.
So far I have checked fuel pump pressure (12 psi)
Read codes from ECM, besides the normal code 12 a code 33 was displayed. This points to the MAP sensor, which I have replaced, I even cleaned out the vacumn source for this sensor, it's attaching hose is intact.
I do not usually replace engine management sensors at will, but the IAC, coolant, MAP and TP sensors are new also, I am desperate.
Evidently there is an issue now with the injectors not firing during startup, can anyone give insight as how to diagnose this? Perhaps no signal from distributor?
Also. What would cause the IAC to allow a huge amount of air bypassed?
Getting back to the TP, I believe I have installed it properly, just to verify resistance varies from.5 volt to 4.5 at WOT, what should resistance readings be for idle to WOT?
Hope someone can help !

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16 Answers

Tiny

Docfixit

June, 5, 2009 AT 1:33 PM

Hi
Think you may need an Ignition control module located in distributor. Must signal PCM engine is cranking to fire injectors on start up.
Hope this helps
Thanks for donate

Tiny

kalanikaau

June, 6, 2009 AT 12:37 AM

Thanks for the advice on the ign. Module, I can surely see how that would affect injector operation.
Can you offer insight as to why the IAC is allowing such a huge amount of air to bypass too?

Tiny

Docfixit

June, 7, 2009 AT 3:08 PM

Maybe carbon deposits. Remove and clean pentil, passage ways. Then plug in turn key on observe pentil does it move? If no then newIAC

Tiny

kalanikaau

June, 7, 2009 AT 6:34 PM

I've already removed an replaced the IAC motor, while I had it out I also cleaned the needle seating area, some how the ECM is telling the motor to open fully (counts)? Or pretty much close to full open because it creates a massive vacumn leak. While I do understand the entire concept of bypassing air around the throttle plates to increase the idle, this leak is simply huge
This is with the engine cold, as it warms up the valve closes somewhat and the engine will actually idle then.
The coolant temperature sensor is new also.

Tiny

Docfixit

June, 10, 2009 AT 9:01 AM

Check out the IAT intake air temp sensor. Located top right center of intake manifold. Check resistence across terminals should be 3400 ohms at 70 deg. Resistence drops as air temp goes up.
Do you get injector spray at start up?

Tiny

kalanikaau

June, 10, 2009 AT 12:46 PM

Will try to locate the sensor (IAT) you describe, do you have a picture of it?
No, the injectors do not spray at startup, would this be caused by this sensor?
I was thinking that the DIS module may be at fault for the no spray problem, do you concur?
Thanks for your time and effort !

Tiny

Docfixit

June, 10, 2009 AT 1:45 PM

Did you have the ignition control module checked?
Not sure what you mean by DIS the PCM grounds injectors when it receives dist. Rotation from ICM

Tiny

kalanikaau

June, 10, 2009 AT 4:00 PM

By DIS I am referring to the distributor assy. Itself.
Are you certain of a IAT (Intake Air Temperature) sensor on this engine?
There is a coolant temperature sensor screwed into the intake manifold (water jacket) adjacent to the t/stat hsg.

Tiny

kalanikaau

June, 17, 2009 AT 6:56 PM

By DIS I am referring to the distributor assy. Itself.
Are you certain of a IAT (Intake Air Temperature) sensor on this engine?
There is a coolant temperature sensor screwed into the intake manifold (water jacket) adjacent to the t/stat hsg.[/Quote: ad2bc5e5f9]

Well, it's been a week since my last question, with no response at all.
Guess you've given up on my problem, I have'nt
Thanks anyway for the tips

Tiny

Docfixit

June, 18, 2009 AT 9:49 AM

Did you have the module checked inside distributor?

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