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So my truck is a ’98 GMC 1500, 2wd, 4.3 Vortec V6, 348,000 km, auto trans.
90% of the time, the truck runs great, except for what I believe is an unrelated issue, I’ll explain in a minute.
So once or twice a week, the truck won’t idle, and start hesitating, overall just running crappy. It seems to be mainly at the lower rpms, seems to clear around 3000 rpm. It usually lasts until it ends up stalling from the low idle which is usually pretty quick if I don’t notice it. It normally starts back up hard like it’s flooded, pedal to the floor helps. Once it starts tho and I give it a couple good revs, it runs just fine. That’s also without turning the key to off the off position. Check engine light came on once with a map sensor code.
So what I’ve done so far:
New map sensor, cleaned egr valve and made sure it wasn’t sticking, and cleaned the throttle body including the idle circuit.
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The other issue is I think is the muffler or cat clogged. It loses power when you shove it to the floor, and the rpms get above 4000 rpm. The shift point on these trucks when they’re on the floor is about 5000 rpm, so sometimes it just stays between 4-5000 rpm, and won’t shift up. My old truck (’97 chev) did the exact same thing because of a clogged cat.
So any guesses?I also checked the fuel pressure today, key on it hits 60, and drops to 55 when the pump stops, it idles around 50 psi, and gets close the 60 when I rev it. 60 is what it’s supposed to be at, but my other truck ran fine on about the same pressure. I bought the truck a couple weeks ago, and the past owner says it has a new pump and filter.
So, any guesses? and Thanks for any help in advance.
And sorry for the long read, and the two posts, seemed easier to read and understand in two haha
[quote=”college man” post=72409]check fuel pressure and FPR.[/quote]
Fuel pressure is written about (added in after), I’m hoping to put the regulator a little further down my list, gotta pull the intake manifold to get to it haha.
The fuel pressure for your truck should be 60 to 67 psi at idle. If possible, I would try another gauge before I did anything else. It takes 60 psi to open the poppet nozzles on the spider fuel injector. The fuel pressure regulator might be the problem.
I’ve always heard 60 is what the pressure should be, and 50 to open the injectors.
Might be the gauge tho, my other truck, ’97 chev with the 305 vortec ran just fine with the same pressure reading. How steady should the reading be? It’s steady at idle, but goes up when I rev it, and it drops just below 50 when I let of the gas while the motor slows back down to idle.The map sensor might have just been doing its job when it triggered a code depending on the code if it was a p0108 it could be indicating that the manifold pressure was abnormally high due to a vacuum leak witch would cause rough idol and stalling
I was going by my information I have here. (Mitchell On Demand) unless it’s a misprint. It could be that your gauge is out a bit. That’s why I suggested trying another gauge.
It was the p0108, but I though if I had a vacuum leak, it’d run bad most of the time, not run bad for a few minutes a couple times a week, and run fine again right after it dies. I never got a maf code either.
[quote=”wysetech” post=72432]I was going by my information I have here. (Mitchell On Demand) unless it’s a misprint. It could be that your gauge is out a bit. That’s why I suggested trying another gauge.[/quote]
If I knew someone with another gauge I would. I’ve been using the one at work on my lunch break haha. I work at an atv dealer/shop, so we’ve got a few specialty tools laying around.
Not Nessasarly it could be a small leak does it happen when u apply a load to the engine also if you had low fuel pressure the problem would be at higher rpm
I’ve only noticed the problem when I’m driving and I let of the gas, normally the idle just drops and it dies, but when I’m able to keep it running, it almost acts like it’s running rich. The motor kinda “chugs” and clears up when the rpms get up, but as soon as it dies and I get it running again, it runs great.
So I still haven’t figured this out yet. When it acts up, it feels just like the truck is running rich and floods out. It acts exactly the same as a carbed engine running with the choke on.
I did find some fuel leaks tho, not in the feed line, but in the return line, and that smaller line from the evap canister.
I don’t figure that’s my problem, but I did notice that the evap line was completely full of gas. I didn’t think that those lines had much for gas in them, maybe I’m wrong tho.
Still no engine light, and I still have the clogged cat issue. When you shove it to the floor, it doesn’t have any power at high rpms, and doesn’t accelerate.I know you said that you serviced the EGR valve but why don’t you disconnect it and try it. It might be sticking open. It might turn on the MIL light with it disconnected but it would rule out the valve sticking.
I thought about that, but don’t they normally turn on the engine light when they stick? My ’97 chev tripped the light when the valve stuck open just a hair, wasn’t even open enough to make the truck run bad.
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