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ok, first i will apologize for such a long post, but i feel its better to give the whole story. i purchased this 1993 gmc sierra 4.3l approx 6 months ago with intent of giving to my son as his first car. it ran when i bought it but has sat since. recently when i went to start it …it will start then immediately stall if you take your foot off the gas…and even if you give it gas its hard to keep running. before i go any further here is what i did so far.
1. rebuilt throttle body, all new gaskets, o rings, rebuilt fuel pressure regulator
2, new cap and rotor
3. new spark plugs and wires
4. new battery as of today.
5. new temp sensor as of today (read somewhere it could cause this situation)
6. verified the correct voltage of the MAP sensor according to haynes manual
7. used volt meter and verifed TPS Switch is working as it should
8. can hear the fuel pump when turning key on.
9. replaced fuel filterI did have a buddy keep his foot on the gas and was able to observe that both fuel injectors were spraying in a normal pattern what appears to be the right amount of fuel.
The distributer doesnt seem to have and play in it, but i am not an expert in them.
I am at my whits end…a few times when it was about to stall i could see a burst of flame come up thru the throttle body, almost like a backfire. I have not checked fuel pressure cause for one i dont have a fuel pressure gauge and two this model doesnt have a schrader valve.
i am at my whits end…and ideas guys and gals?
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The fact that it dies when you lift your foot off the gas tells me that the idle air control valve is not functioning properly… Is it part of the throttle body?
Also, have you checked compression in all the cylinders? Have you checked the fuel pressure? Hearing the pump may not be sufficient…Hope that helps!
I have not checked fuel pressure because it does not have a Schroeder valve and I am without a fuel pressure gauge. I will have to secure an adapter to do so. I did not do a compression test but I will now tomorrow as I do have a compression tester. Is there a way to test the idle air control valve?
I think there might be…but it probably requires taking it off… All it does is allow small amounts of idle air into the manifold when the throttle is closed. Typically a plunger that opens when voltage is applied. When you take it off is usually when you will see oil and crud, so generally they just need cleaning.
Ok, i cleaned the IAC today, it was pretty dirty but did not change the symptoms. I ran the compression test and got the following results.
cyl 1 165
cyl 3 160
cyl 5 165
cyl 2 160
cyl 4 165
cyl 6 165are these good values for this engine. the 4.3L?
I have not checked fuel pressure yet as i don’t have the adapter or the gauge yet.
I also checked my spark plug wiring to make sure i wasn’t shooting myself in the foot with some dumb mistake.
Any other ideas before i check the fuel pressure? Does anyone know of a way to check the IAC to see if its bad?
One last note i forgot to mention in my original post is that the check engine light is not on.
thankscheck and or replace the ground wire/strap from firewall to frame on passenger side of the truck you will see it bolted by the upper control arm at the frame. they have a tendency to corrode. also look at the EGR valve if it is sticking open it could cause the symptoms.
good luck and let me knowI did check the three grounds with a volt meter. I put the positive on the battery terminal and checked both sides of each ground point. The ground going from engine to firewall, the one going from frame to firewall ad one going from battery to fender. All over 12 v on both sides. I will check the egr valve tomorrow. This is kicking my butt. Lol
put a jumper wire on the firewall to battery ground and see if you get a “arc”. the test you performed may give you a false positive.
ok, all grounds are good, verified with jumper wire. i also checked the egr valve, it was bad (did not hold vacuum)..i replaced it this morning, same result. while out at the auto parts store i rented a fuel pressure gauge. when i hook it up i get 2psi when turning the key to the on position,,but it doest hold at approx 2 psi.it goes up to 2 psi then drops back to zero. i clamped off the return line with same result. I believe the pressure should be between 12 and 15 psi. So my question now is, do i have a bad fuel pump or did i screw up the pressure regulator when i rebuilt the throttle body…as part of the rebuild kit was the diaphragm for the pressure regulator…and it was a PITA to put that on with the spring. In my head i would think that if i clamp off the return line it should hold at least that 2 psi, right?
fuel pump on those systems need 13 psi min cranking and running, pinching off the return line you are dead heading the pump and should shoot the pressure to around 20 psi or higher. 2 psi is too low the truck wont run period.
If the engine is off then the fuel pressure should hold. If it drops, there is a leak somewhere. If the injectors are leaking that could be the cause of the problem.
Did this issue occur before you rebuilt the TBI? If not then the rebuild may be the cause. Those old TBI systems were tricky.
If the problem was there before you did the rebuild then perhaps you have a vacuum leak or possibly an issue with the timing. Both of those can cause performance problems such as what you describe.
More info on solving performance problems like this here.
http://www.ericthecarguy.com/faq/solving-automotive-performance-issues
Good luck and keep us posted.
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