first let me say thank you for this forum ... and secondly as i have seen many post of the rough idle i too have that same problem ... this truck has acted this way since i purchased it. At times the check engine light will come on but never stay on. I have tested it numberous times and even took it to GM to run a diagnostic check... everything comes back fine ... i have removed the egr valve and cleaned it although it was in decent shape anyway. I get terrrible gas milage also ... I think my next step will be to replace the FPR. As of right now the freaking thing wont even start now ... ugg ... the first good snow of the year and i dont have my 4WD ... any suggestions will be greatly appriciated .. peace ... ps.. 93 ... 4.3 vortex

 

You can test your fuel pressure to narrow down the problem to the fuel system. You could also remove the upper plenum, pressurize the system, and inspect for drips/leaks.

What things have you done or had done to try to repair/diagnose this condition?

 

wasnt really worried about the pressure do to the fact that when i check the releif valve there is lots of gas that comes out ( what i call the relief valve anyway .. looks like a valve stem plug) .. as i stated.. the diaganostic test comes back nothing wrong ... i tried the map sensor anyway ( do to my buddy works at autoparts store and he let me try if for free ) .. no effect .. havent proceed to replace any other sensors since the test always come back fine ... found a mechanic how told me it was prolly most definately a clogged EGR valve.. but i removed and cleaned it .. it was in decent shape .. normal carbon nothing drastic.... just going by what i have found on here for future solutions ....

 

Diagnoise the problem first THEN replace the part. It is cheaper this way. Start with the fuel pressure

 

Ok, let me see if I got this right.

You have a rough idle?
You bought this truck with the rough idle?
How many miles are on the truck?
When was the last tune-up?

A rough idle could be from a host of things.
Start with the basics, and in this order.
Check for false air getting into engine. False air is air that does not go past the MAP sensor. Such as a broken vacuum line, disconnected vacuum line, etc.
Give it a tune-up, plugs (AC DELCO), plug wires, cap and rotor, fule filter, air filter, pvc valve and a fresh oil change. Wouldn't hurt to run some kind of oil treatment through the engine to clena out the sludge. Do this before the oil change. I prefer Marvelous Marvins Msytery oil, other like Lucas, or Seafoam.

 

I have and always will perform the general maintance on my vehicles... a general tune up is always the first thing i do when i have purchased a used vehicle... this problem extends past your basic tune up ... and how are you gonna diagnose the problem when all diagnostic test show nothing is wrong ??? I wouldnt be here looking for answers if the problem was obvious ...

 

When you say "all of the diagnostic test", what have you done? I asked the question and you still haven't answered. How do you expect us to help you find this 'mystery' problem if you can't even answer that. I for one do not like stating things if you have already done them. See, you never even stated that you did a tune up. That information would have helped greatly. We do not know you personally, nor do we know what you are capable of and what experience you have. Please help us help you.

Also, 'lots of gas' does not mean that you have the proper pressure at the fuel injectors. The ONLY way to test the fuel system is with a fuel pressure gauge connected to the fuel pressure test port.

I'll also ask how you have been doing all of the diagnostic testing. Do you have a scan tool that has bi-directional control that you are using?

 

the diagnostic test i have been refering to are the hand held computer ... like the ones at say auto zone ... and when that came back everything ok .. i took it to GM and they put it on their diagnostic machine.. it came back everything ok ...as far as i know if there was aproblem with a sensor or such GM's test would have caught that right ? i thought i had stated that b4...and sry... im probably coming off a little sarcastic .. but to c... diagnose the prob ... Then in emphasis .. fix it ... really was unnessicary to me ..if im not mistaken the fuel test port is the valve stem type port at the end of the injector feed tube correct .. and if so lots of gas means it sprays when i push in on the needlewhen it it running ... ... .. the truck has 125k on it Hanr3

 

Lots of gas a pressure reading is not... (sorry for the yoda like comment) Fuel pressure is one of those things that is required for a properly running engine. Can you tell the difference between 50psi and 60psi by pushing on the schrader valve and watching how far it sprays?

Being a '93, the computer based diagnostics are limited in what they can and cannot do. I temporarily forgot that we were talking about a '93 when I made the comment on the scantool with bi-directional controls. An OBD1 system will tell you if a sensor has fallen outside of its limits, but not when it does so.

Your rough idle could be caused by any number of things. Since you have already performed a full tune up consisting of (I assume) plugs, wires, cap and rotor; and you have cleaned the EGR valve as well as replaced the MAP sensor, I am going to ignore them for the time being. I'll list the other posibilities.

If you still feel this is a fuel problem and do not wish to test the fuel pressure, you can pull off the plastic 'vortec' cover on top of the intake plenum, then remove the manifold tuning valve (round plastic peice) and have a look down inside of the intake. If you see fuel sitting down in the bottom of the plenum, then you have an internal fuel leak at which time you will need to remove the entire upper plenum to inspect where the leak is coming from. Now you will only be able to see down into one side of the plenum by removing the MTV, but this should give you a good view of the fuel pressure regulator side which is quite common on the CPI motors.

A fuel pressure gauge would #1 tell you if you have the proper 55-60psi for the starting of the engine and #2 be able to monitor the fuel pressure after you turn the key off (fuel pump off); if the pressure drops, you have a fuel leak and from there you must determine if it is internal to the fuel tank (unlikely), external (should be obvious), or internal to the engine (most likely).

And that is all assuming that it is a fuel leak.

Moving on.

If you crack the throttle manually (foot) when attempting to start the engine, does this help? If so, I may be inclined to remove and clean the IAC valve and the bore it rides in. It is possible that the idle air control valve in the throttle body has buildup around it, restricting its range of motion to a point where it cannot open enough for proper air supply while starting the truck.

Going even further.

It is possible that the base timing is off... If all else fails, a timing light will verify that the spark is happening in proper time with the rest of the engine. There should be a wire inside the truck on the passenger side of the transmission tunnel (green if memory serves me) that can be disconnected to deactivate the electronic spark advance as controlled by the ECU.

Back up for a moment...

More along the lines of a tune up. Due to the age and mileage of the vehicle, it is permissible that the O2 sensor could be quite lazy. On the newer OBD2 vehicles (96+), the sensor values can be watched in real time from a scan tool. On the older OBD1 models, this was not possible and an oscilliscope must be used by tapping into the wiring. The OBD2 monitors are also a lot better at picking up a lazy sensor than the older ECUs. In this case, I may be inclined to replace the sensor. For your truck, an AC Delco O2 sensor will run you ~$80 (AC Delco PN 213-632) for a 3 wire heated O2 sensor and less for the single wire sensor. It depends on what your truck is equiped with.

 

Sorry if Swart posted this, however I dont have time to read through his post.

OBD checks the sensors and reports back its findings.
False air-will not show up as a problem.
Fuel pressure will not show up as a problem, OBD does not tet fuel pressure.

Being a '93, do you ahve a CPi or TBI engine?


By the way, I have no idea what skill level or mechanical expierience you have. Nor do I know what your standard practices are concerning purchasing a nused truck.

I ahve rebuilt several transmissions, however I still ask questions.