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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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[quote=”wysetech” post=72904]A vacuum operated valve seldom sets a code when it sticks but an electronic valve is more likely to set a fault code if it sticks.[/quote]
This one is electric.
I didnt relize u said U had a clogged cat that will def cause a lack of power and I know u said u changed the egr but The new part could of been defective A stuck-open EGR will also cause a stall when coming to a stop- that is, if you’re stopping after a “high speed, low throttle” situation like driving on a freeway When the car stalls, the EGR valve closes, and then the car runs normally until you get up to a high speed/low throttle situation again.
Sry just relized u didnt replace the egr you cleaned it but I would def take another look because it sounds like that’s the problem
I never replaced the egr valve, I just removed and cleaned it, and made sure there wasn’t carbon build-up causing it to stick, and made sure it opened and closed easily and smoothly.
I might “fix” the cat this week sometime, but I might wait until I replace my muffler first. It’s not leaking, but the baffles inside are loose and rattling, and I’m thinking one might be out of place, restricting flow. I don’t figure that’s the case, but I need a muffler anyway, the rattle drives me crazy, and it makes the truck sound like a tractor haha
But with that evap line going to the tank from the canister, aren’t those more for fumes? I had what seemed like a complete line full ( or more) of gas run out while I fixed the leak in the line, and it was still running out until I got the clamp tightened.
There shouldn’t be fuel In The evap line I know on some vechiles if u fill up the fuel tank to often it can get fuel in the canister and saturate it and the charcoal can get in the fuel lines
Since the evap vents into the intake, is there anyway it could be venting gas in, causing it to run rich like it does? Or feels to at least.
And I also looked at one of my spark plugs last night, they’re not that old, but they’re not AC Delco’s, so I’m still going to swap them out at some point haha.
I fixed my second fuel like leak last night, this was the one in the return line, the other leak was that evap line.
I pulled the canister as well to see if there was any gas in it, and there was some, enough to hear is sloshing around when I shook it. I’m guessing that little bit is normal?
I’ve noticed that the truck acts up at about the same place while going to work, almost every morning, and seems to be a little further down the road on colder mornings, so it’s something to do with the engine temp triggering something. It happens after almost 5 minutes of driving.I’ve noticed that the truck smokes when it acting up, it looks like blue or black, but it’s hard to tell with the sun shinning from behind me, and while I’m trying to drive in busy traffic haha
So I finally got this figured out. The problem was with the evap system, and when it got into the purge cycle, it would suck in straight gas.
For now I unplugged the electrical connection from the purge valve, and unplugged the vent line from the tank off the canister so the tank can still vent.
So now the truck is running great, doesn’t even bring up and engine code. Still gotta deal with that clogged cat tho.Hmm, I’m not sure you’re quite done yet.
I would want to figure out why the canister is full of gas.
Perhaps the vent valve was stuck closed for a few months, allowing the canister to get saturated? Now that it’s venting into the engine, that might be just the right thing to have happen. You want to burn off all that fuel in there, or you could get a new canister.
Or if the vent wasn’t stuck, something was pushing too much gas into the canister, more than could be vented in a reasonable amount of running time.
Or if you always did very short trips, the canister would never go into its vent cycle, or not for long, causing it to fill up faster than it could get vented.
I don’t think I’d like to be driving a lot with a soaked canister. Not only potentially smelly but a bit of a fire hazard perhaps.
I had the canister off, and it didn’t seem to have much for a fuel in it. I do get some gas coming out of the vent line sometimes which I didn’t think was right. Usually around the same times the truck would act up tho, so that defiantly had something to do with it.
I started concentrating on the clogged cat, and I’m thinking now that might not be the problem. It still loses power at high rpm, but it doesn’t run as smoothly when it hits it’s rpm “cap” like my other truck with a clogged cat did. In neutral it’ll hit about 3500, and start surging a bit, and sounding like it’s misfiring.
I took it for the drive, and held it to the floor for a ways to see how hot the cat got, it seemed pretty warm, but I also got 4 engines codes. 1 for random misfire (P0300), and 3 O2 sensor codes (P0131, P0143, and P0151). I’ve heard a lean condition can cause this, so I’m thinking maybe a weak pump or regulator.
I know a clogged cat can cause O2 sensor codes as well, but that can also cause a MAF and other codes which I have yet to see.
I was going to buy a vacuum gauge and do some tests to check for a clogged cat, but neither of the 2 canadian tire’s in the city had one in stock.Also the truck seems to make great power at the low rpms, and doesn’t have any other performance problems. It starts very easily, in a second of cranking or less, and that’s going straight from the off position to the start position, so not letting the fuel pump build up first.
I think I’ve narrowed this down to the fuel pump and not the cat. I got a vacuum gauge, and measured vacuum. With the rpms steady at about 2500, the vacuum was good, and stayed there, and didn’t drop off.
I can’t remember how much vacuum it was pulling, but the gauge I was using says on it what the normal zone is, and it was in it. I put in new AC Delco plugs, and swaped in a different coil, along with the new cap and rotor I put in a few weeks ago.I found a cheap fuel pump online, (prime choice auto), for $55 plus tax, free shipping. I think the local prices are around $200-3.
I know it’s probably not going to be a long lasting pump, but I’d rather put in the cheap one and find out that’s not my problem, just in case it isn’t.Yes, it hits 60 key on, the drops to 55 when the pump stops. Idles at 50 psi. It won’t rev past 3500 rpm and starts misfiring a bit and surging, the fuel pressure surges above and below 50.
These trucks are supposed to be at 60 psi, 55 at the lowest, and I believe the popet valve injectors won’t open below 50.
I also put in a new fuel filter, and no difference.
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