Hi guys, first time I've ever tried coming online to ask someone because I'm completely stumped. Disclaimer: I am NOT a mechanic, have just figured it out by ear my whole life. My vehicle: 1996 GMC Sierra 1500 2WD Auto w/ 5.7l Vortec Beginning: Just bought the truck for next to nothing with bad engine. Replaced engine 1 month ago with identical engine with "140k" miles on it. (Who really knows right... it looked and ran great for last month.) My buddy and I split the work and turned it into a few hour process. In doing so, we somehow managed to completely miss plugging the map sensor back in this whole month. Middle: The truck ran strong, minus a very quick and sudden complete stall, if you stepped your foot in the gas. The stall actually felt like the truck just "died" for about 1 second, then VROOOM. The truck also smelled like it was burning lean, but I never had time to tinker with it. About 2 weeks ago, I started to notice that the truck would run kind of rough and vibrate/shake any time you were "coasting" down the road and barely had to touch the gas petal to remain at 30mph/45mph/55mph for city speed limits. It also felt like the truck would shift too early and sort of bog down. Today, I drove the truck for about 10 miles and on the way home, I noticed that the truck would do that "pipe pop" like a manual transmission when you rack up the rpms and let off the gas. (I have dual exhaust from manifolds to 2 flowmaster mufflers and echo tips, no cats). Anyway, normally, if I was going 45mph in 3rd gear, and let off the gas, it would seem like it went into "neutral" and quietened down. Today, it was doing the "pop pop pop BOOM pop pop" sound. It actually backfired a few times. When I noticed it do it by itself, I got curious and actually put it into 2nd manually, to see if it would do it again. It did. I NEVER abused it and the RPMs never went over 2.5k in the process... I think it redlines at 6 or 7... After I got it home, didn't think anything about it, because it was cold this morning, cold start, left quick.. Ending: Later, I took the truck again, but made it about 8 miles before I just lost all power. I was on the interstate, attempting to reach 75mph and keep up with traffic when it just progressively lost power. By the time 5 more miles were up, I was on the interstate going 50mph with my foot almost in the floor. So I let off the gas and putted to the nearest exit, where I parked it at an autozone. I had autozone do a diagnostic scan, and it said MAF and MAP sensor. (This is how I found out the map sensor had never been plugged back in) The truck would start, idle poorly, then eventually adjust up to being able to idle fairly... but the moment I put it in gear, it just bogs down and loses all power. I have another vehicle (98 z71 w/ same motor) so I changed the MAP sensor, the MAF sensor, the idle air sensor and throttle position sensor with the other vehicle and no difference other than idling more poorly with different configurations of swapping parts. (I did not do battery reset between sensor testing). I tried changing the entire breather and no difference. I had recently replaced the fuel sending unit and the guys that did my exhaust removed my cats and wouldn't let me keep them...... I have not tried crank sensor, cam pos sensor or anti-knock sensor. Tonight after dark, I went back out to the truck to tinker and noticed that I got a neat little display show from 4 plug wires AND my coil wire going to distributor. The second and last plugs (from front) on BOTH sides, were jumping. Not a strong jump, just enough to show a fuzzy blue light show. The connection at the coil was doing the same thing. So #4, #8, #3, #7 and coil. Everything was closed, so I couldn't change plugs and wires yet. So right now, my truck idles at almost correct state, but vibrates. The moment I attempt to put it in gear, it bogs way down and tries to die, and shakes horribly. If I try to drive, I have no power. With petal 1/2 way down, I would hit 30 mph in about 4 minutes... Could it just be the plugs/wires? Is it something more simple? Until I noticed the sparks, I was going to try fuel filter next. Or thinking I got water-gas... Any ideas on what to try next? Sorry for the long post, just trying to be thorough in the explanation.
Yes it could be just the plugs wires and dist cap. I'm surprised the check engine light is not on because of the missing cat.
Be sure and clear the codes after you replace the ignition parts.
SOURCE: 1998 GMC Sierra- runs rough then stalls when
had this problem on my neighbors 98 chevy truck with the same engine !!
it turnd out too be the fuel pressure regulator,you have too pull the upper intake too replace it!!
SOURCE: rough idle / varying oil pressure when accelerating
Ok not to keen on newer vehicles actually prefer the carburated ones, but oil pressure is supposed increase on your gauge when accelerating. If it wasn't going up before and just staying at 40 the needle might have been stuck. But idle should be 40 lbs and when you work the moter (reving or driving down the road) the guage should show an increase for every 1000 rpm above idle. Second if you can get a hand held scanner (bought mine off ebay for $30) check for any codes in your computer you will probably find that you either have a bank running wrong or some other sensor that needs to be changed. I'd go with getting my own scanner it saves alot of money and time, garages make their money here.
SOURCE: Truck losing power and service engine soon light on
Like to other guy said, give it a tuneup. Also try SEAFOAM to clean carb up of slug. Also check to make sure the butter fly for the choke has not come loose, causing it the choke it down. Failed choke pull-off.
SOURCE: I have 96 gmc 350 vortec. missing really bad. Ive
ou need to check compression on all the cly. do you know which cly is dead?
SOURCE: 2001 Tahoe: Hard to start, idles rough, check engine light on.
Flashing check engine light is missfire, fuel pump probably causing multiple tries to start.Having to build fuel preasure,vac leak on one side of intake probably causing rich/lean condition.Also check back preasure to cat ,Sound like you are moving down right track.Mas air flow would probably set lean code bank1 and lean code bank2
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