Mechanics

5.7L VORTEC - NO POWER IN GEAR, GOOD IDLE

1996 GMC Sierra • 140,000 miles

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?
Avatar
96GMCSierra
June 10, 2013.




With all that poping in the in intake and the way you say is running. I would remove both valve covers and check if the cam for worn lobes. Watch as it runs for tappets not fully moving. Then you can remove the lifter to inspect it. Good luck I hope this helps

Cadieman
Jun 11, 2013.
To clarify, the popping sound is coming from the exhaust, just to demonstrate the similar behavior to a manual transmission vehicle's sound when you leave it in 1st gear, hit 4k RPM and then let off the gas. How it slowly goes back down RPMs and goes "pop pop pop pop" as it's doing it.

There is no popping sound coming from the intake/engine at all, except the typical clinks of an engine thats JUST about ready to die. (The sound ALL movies make when a car breaks down while still rolling.) And that only happens when I put it in gear.

Should I still check the cam?
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Tiny
96GMCSierra
Jun 11, 2013.
Any other comments or ideas?

Tiny
96GMCSierra
Jun 11, 2013.
UPDATE: Replaced spark plugs/wires and tested each for adequate spark and found something strange. We were able to remove ALL spark plug wires from the passenger side and there was no difference in how the engine ran. So #2-#8 cylinders are not working. Where would I even begin?

Tiny
96GMCSierra
Jun 11, 2013.
Are two and eight on the same coil? Do a compression test on the motor. Then come back.

Cadieman
Jun 12, 2013.
?.

It's ALL FOUR cylinders on the passenger side. #2, #4, #6, #8

I also have only one coil.

They have plenty of compression.

Tiny
96GMCSierra
Jun 12, 2013.

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