Hello,
A friend of mine has some problems with his 1993 GMC 1500 4.3/5sp. The engine will not rev past 2000RPMs under load, but will in neutral. There is also a great power loss. The truck has 200,000miles and has run fine until now. He's checked fuel pressure and vacuum, both seem to be OK. He suspected a vacuum leak, so he replaced the throttle body and intake gaskets. The cat has been removed, so that's been ruled out as well.

To me it sounds like a fuel issue, but like I said, fuel pressure appears to be OK.
Any ideas?:confused:

Thanks,
Mikel

Check the compression on all cylinders.

I have the exact problem at the moment and it was hard to find. I had good gas pressure on the fuel line going into the engine but in the engine I had no fuel pressure with the engine running at any more than idle. Its bad injectors and it turns out these are infamous for shelling injectors.
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Sandking,
The engine doesn't burn any oil, and in order to lose enough compression for such a power loss, I would expect oil usage to be massive.

Comeonstart,
How many injectors failed in your engine? I suppose one out of six wouldn't cause such a dramatic power loss, and the odds of more than one failing at the same time are pretty slim.

Thank you.
Mikel

loosen the downpips and make sure the cat isn't clogged. a bad lifter will do it too.

Sandking,
The engine doesn't burn any oil, and in order to lose enough compression for such a power loss, I would expect oil usage to be massive.

Comeonstart,
How many injectors failed in your engine? I suppose one out of six wouldn't cause such a dramatic power loss, and the odds of more than one failing at the same time are pretty slim.

Thank you.
Mikel

All my injectors went bad, evidently some nasty crap got into the gas lines. It doesnt matter really, as my mechanic told me that to replace one you have to replace them all.
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All my injectors went bad, evidently some nasty crap got into the gas lines. It doesnt matter really, as my mechanic told me that to replace one you have to replace them all.

Before you go replacing them pull out your injectors and take them to a fuel injection shop. If they are wrecked they will tell you and it should be no charge. If they are worth fixing they will clean and/or repair and flow test. Price is a lot cheaper than buying new ones.

A 93 will not have 6 injectors probably just 2 if it is throttle body
if it is a CPI then one injectors and 6 pop-offs
if it is throttle body look at the injectors at 2000
and see if they flow good

did it get a new fuel filter?
it could have good pressure but not accurate flow at 2000

take it to a shop and save money in the long run instead of throwing parts at it until it is fixed.

take it to a shop and save money in the long run instead of throwing parts at it until it is fixed.

Yeah because telling people to go to a shop is what we're all about here. :shaking: He is asking how to troubleshoot the problem. He's replaced a couple of gaskets, if that'll break the bank then he needs to get a bus pass, not go to a shop and have them throw parts at it at $90/hour.

you might take it to a part store and have them put it on a dime store scanner. One possibility is a bad knock sensor or a light knock starting. Under slight load you may have a light knock causing it to cut power dramaitacly so as to save the motor. My wifes Jimmy has the same motor and it has a slight knock under revs and it cuts the power and wont down shift (swapping the motor now).

90% chance its your fuel filter.
It will have correct pressure at idle but when the fuel demand goes up----
Take it off, you should be able to blow air through it with your mouth. If you cant, its shot!!
I replace mine on my 2000 C3500 every 10k
Cuz at 15-20k My truck does what yours is doing!!

I'd start with the easiest stuff, maybe fuel filter....but the first thing i'd check is the ignition. I'm pretty sure those had distributors, take the cap off and see what shape everything is in, spray the plug wires down with water and see if it causes a stumble, pull the plugs and see what condition they're in, all that stuff...

Hello and thank for the responses.

The fuel pump and fuel filter were replaced recently. Thinking of bad injectors, I'm going to tell my friend to start pulling plug wires to see if he finds a cylinder in which it makes no difference. I'll make sure he checks the ignition parts and timing as well.

Fixed! It was a fuel line blockage. Thank you all.