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1996 5.7 vortec running poorly


boscat43
05-19-2008, 11:23 PM
I am new to the forums but it looks like a busy place so I thought I would spill my problems and give it a try. My 96 5.7 is in a 2500 truck with about 150 000 miles and is missing and getting very poor milage. I have had it at 4 different shops(2 Gm dealership and 2 private) and no one seems to be able to help.

Shop 1 (private shop) I took it there cause my check engine light was on and had no power. Missing down low and surging at higher rpm. code showed up as O2 sencor so he replaced, ran o.k. for 2 miles, light back on. turned around scanned it said bad MAF, TP, and two more O2 sencors. I had a hard time believing so cleared codes and left. Ran o.k. for a few 100 miles

Shop 2 (gm dealer) told me that the timing chain had skipped for sure and that there was no use doing anything else till that was fixed. I left there knowing that there is no way on this earth that the chain had jumped cause these motors have a plastic cover that would chatter if that where to occur.

Shop 3 (private shop) Installed his scanner, which shows no codes or any miss fires of any sort, yet the truck dash is shaking so hard at idle I cant even play a cd. He said he seen it lots and laughed at the gm dealer for not figuring it out, it was the distributer for sure with out a doubt. Installed new distributer, not one little bit different. Wont return my phone calls (suprise)

Shop 4 (Gm dealer) Scanned it, nothing electrical showed up, told me low vacuum at intake, cam lobes gone for sure. I take it home pull the motor, looks like brand new inside. Call them back, wont talk unless i bring the truck back. 107.00 an hour they charge. I guess they will talk for that much. Cost me 347.00 just to put it on their scanner.

Here I sit now. I have about 3400.00 into the truck now, and it still runs really poorly. It starts good, misses slightly at idle on passenger side, no power, getting about 6 mpg, mid range it bucks around, surges at higher rpm. At 60 mph in OD it will not gain any speed until you push it far enough to make it shift down. If you dont push it far enough the dash and everything shutters real hard, been running it without a hood and I can see the engine flipping back and forth if you put a load to it and dont let it rev.

A friend of mine let me borrow his truck for the weekend, so I went to work. I swapped everything from his to mine and mine to his. Injectors, MAF, EGR, Top part of intake and all sencors on it including throttle body assy. Cam sencor, basicly every thing that I could without pulling motors. His keeps running good, mine, well...

here is a list of what has been done or tested
compr 150. psi all dead even, cyl leak down 8 to 10%. the following are new parts--- plugs cap rotor distributor timing chain new intake gks plug wires fuel filter cat convertors, 02 sencor, ign coil. I tested fuel press

I do believe that it is more then just one cyl missing it has to be something else that i am missing. I took the good running truck and pulled one wire to simulate one cyl missing, still had way more power and ran smoother than mine. I think I would have to pull 3 or 4 wires. It also isnt a miss that you would get off one cyl, as it has no rythem to it, the miss is all over the place. One more thing, my charging light comes on at high rpm, 4000 and above, cant really explain that but might be related?

If any one has any ideas, let me know. Any help would be great, other then that, if any one wants to buy a good truck, I might have just the deal for you.
Thanks

2000CAYukon
05-20-2008, 11:52 PM
Can you post what the fuel pressure was? Also what does the fuel pressure do when you shut off the engine?

//2000CAYukon

ponyracer
05-21-2008, 12:00 AM
Hi, I'm new here too. Do a cooling system pressure test and spray the intake for possible vacuum leaks. I've seen these have leaking intake manifolds. I've also seen the fuel pressure regulators leak under the plastic upper intake. Hope this helps.

boscat43
05-21-2008, 07:58 AM
To 2000CAYUKON; I have done a press check on the fuel but I forget what the actual numbers where. I know they where within speck from the book, and the press stayed for a while after shutting vehicle off. I will check it again tonight though and give you exact numbers.

To Ponyracer; That is something I have not done. I will also press check my cooling system tonight. I do know that my anti-freeze is not going down, and less then 20 miles ago I replaced the old intake gsk with the new metal embosed ones from Felpro. As for the press regulator, I think I eliminated that when I took the complete spider assy from the truck know to run good, onto mine, and vise versa, good truck kept running real good, mine did not.

Thanks for the help guys and I will post some results later tonight.

2000CAYukon
05-21-2008, 10:09 AM
Did you swap distributers as well? The drive gear can wear and it will cause issues. How about the PCM? You could be dealing with a bad one.

//2000CAYukon

boscat43
05-21-2008, 08:20 PM
I realy want to sound intelligent when I am on here but sometimes it just doesnt work that way. Could you spell out what you are refering to when you say PCM? If it is the main computer sitting on the fenderwell, I have tried that. Also the distributor is a brand new one, I even spent the extra 50.00 bucks to get a new instead of a rebuilt one.

Thanks again.

2000CAYukon
05-21-2008, 08:47 PM
I realy want to sound intelligent when I am on here but sometimes it just doesnt work that way. Could you spell out what you are refering to when you say PCM? If it is the main computer sitting on the fenderwell, I have tried that. Also the distributor is a brand new one, I even spent the extra 50.00 bucks to get a new instead of a rebuilt one.

Thanks again.

PCM is the Powertrain Control Module which is the main computer. Did the new dist come with a new drive gear (bottom of the dist) or did you use the old one?

I believe you replaced the cats but was a backpressure test done? Just trying to rule out a blocked exhaust especially if old of the old cats broke apart and clogged the muffler.

If the truck is put in 1st gear, do you still have a misfire at midrange RPMs?

Does 3rd gear work better at 60 and will it go higher? Just wondering if this is a tranny issue not an engine issue (except if you feel a mis at idle, not in gear).

//2000CAYukon

2000CAYukon
05-21-2008, 09:33 PM
Couple of other questions:

1) What brand plugs, wire, cap, rotor, dist?

2) Was a scan tool used to set the cam retard? (Timing can not be set with a timing light).

//2000CAYukon

boscat43
05-22-2008, 07:31 AM
So I guess I did change out the PCM. The only thing I found weird was that the check engine light came on when swapped. Only on my truck and not on my test truck. The test truck still ran perfect with my PCM. The distr. did come ready to drop in, it even came with a new cap and rotor again. After I installed it I did have to take it to a shop with a scanner so he could set the timing. It was kinda funny cause as he had his scanner on there and had it on the display where it shows all cyl misfires, I put it in drive as to do a burn out. U take it to the point of really lugging the motor but not spin the tires, the motor is just about to take flight it is shaking and missing so bad. His scanner is sitting there telling us there is no miss fire and never has been in the history part of it.
So I guess that kinda answers the question about first gear. In third gear it will go faster but at higher rpm it feels more like it is surging back and forth instead of really feeling the miss.
I never did do an actuall test on the exhaust. Here is actually what I did with the cats. I just cut them open and gutted them. When I had them cut open, I ran the truck. The miss was really easy to pick out then cause The tone of the exhaust would change so much. I would think that should rule out plugged exhaust.

Brand of plugs are NGK. Wires are also NGK. Distr N cap n rotor I am not sure of the make, something sure fire. Here is also something that I forgot to mention earlier. When I started having this trouble about four months ago, the miss was for sure on the driver side. I can tell this cause my dual exhaust is a true dual, with no H pipe joining the two. U could put your hand over the pipes at the same time cause they come straight out the back and feel it kinda kick though the exhaust. Now after all this messing around, it is now missing on the passenger side. It switched sides after I had the motor out and inspected it, so before I got this other truck to pull the pieces off of.
My question is this. I am dealing with something so simple as bad spark plugs, as that is the only thing that I can think of that would switch side to side? But on the other hand, should that not show up on a scanner? I forget the number of the plug, I probably just the standard plug that it would call for.

2000CAYukon
05-22-2008, 08:01 PM
I looked at similar posts on a few forums (including this one) and the common issues are:

Crank Position Sensor
Leaking Injector (There is only 1 unless you upgraded to the new spider with 8 injectors).
Worn Dist and/or dist drive gear
Leaking Fuel Pressure Regulator
Poor fuel pressure
Bad plugs/wires/coil

I don't understand how the scanner did not show misfires when the cam retard was being set and the engine was clearly missing.

It is hard to tell if this is fuel related, electrical or mechanical (although compression test and leak down looked good).

//2000CAYukon

boscat43
05-23-2008, 07:42 AM
Well I think I might have a solution to my problem. I just bought my buddys truck so it looks like i will be doing an engine swap in the near future. That will give me a chance to completle tear down my old motor and see if there was something that i missed mechanicly.

What if I do a swap and it still runs poorly.......

2000CAYukon
05-23-2008, 10:12 AM
Well I think I might have a solution to my problem. I just bought my buddys truck so it looks like i will be doing an engine swap in the near future. That will give me a chance to completle tear down my old motor and see if there was something that i missed mechanicly.

What if I do a swap and it still runs poorly.......

Then it has to be fuel system (pump, relay, ground, wiring) or an electrical issue (bad ground, bad wire, bad connection somewhere).

//2000CAYukon

boscat43
05-23-2008, 10:11 PM
It might be a week or so but I will try to post the results of my engine swap. Here is to good luck to me!!

kdbout5
10-15-2008, 02:05 PM
Believe it or not a bad alternator can cause what your describing. If the diodes are bad it will produce alternating current. Your DC designed vehicle does not like alternating current. It may not be the cause but worth a look.

sechracer
10-17-2008, 05:48 PM
these trucks have funky dist caps on them. Take the one off your buddies truck and try it on yours...... Mine did the same and acted the same with no codes. It ended up having a hairline crack on the inside and was shorting itself out. I put a brand new delco one on it and havent had a problem for almost 2 years now.

tempfixit
11-15-2008, 04:05 AM
boscat42 did you fiqure out what the problem was. Very interested to find what the problem was for future reference.

Thanks

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