95 runs really bad at highway speeds

scavanger · 10-24-2009, 12:44 PM

95 GMC Z71 5.7 auto. 4x4
symptoms: runs pretty good cold. Once it gets to open loop things go bad. At highway speeds it will act like I have a crossfire (maybe not hitting on one causing crossfire?) It will suddenly run good for about 10 seconds or so and then do it again. I have noticed that it will backfire a little.Starts very well, and never dies. I haven't checked fuel pressure yet going to make a fitting to replace fuel filter. (priced on at parts store.$79

) With long hose I can check going down the road.Plugs,wires,cap, and rotor aren't to old.Also wires not oily,I keep engine compartment clean.The plugs do show rich,, to the point of being black.
A few months ago I had number 7 plug wire come off and it ran something like this. Could I have burned through a plug wire ? Bad coil , would it start well cold and hot ? Inside of cap and rotor look good. Right now I am going out to check compression if I can find my gauge, been long time since I used it.
Any help would be appreciated.

Scav.

Case 7230 · 10-24-2009, 01:36 PM

My guesses are bad Engine Coolant Temperature sensor, bad Fuel Pressure Regulator, bad oxygen sensor, bad EGR valve or possible restricted exhaust flow.
Might get the paperclip out and check for codes?

scavanger · 10-24-2009, 02:45 PM

Forgot to mention about 3 weeks ago I got a O2 rich.

I did a compression check with my crappy gauge. All over 100 within 10%
How sensitive are these ignition systems to plug gap. When I put the plugs in ,15000 miles ago, I must not have been paying attention and gapped them at .045 instead of .035 .

I gapped them to .035 ,warmed it up and took it out a couple miles. Was fully warmed up and didn't show any signs of misfire .Not sure if this tells me anything because sometimes it needed to be hotter to effect it. I have never seen anything that sensative to plug gap. Wondering if a slightly weak coil would do this.

Scav

BernieB · 10-24-2009, 02:51 PM

With plugs fouled from a rich mixture, yeah, that .10 could make a difference in the spark.

scavanger · 10-24-2009, 03:08 PM

So then I need to concentrate on fuel mixture or bad coil. I ohmed the wires all looked good.
This would not be a heated O2 sensor , Is there a ohm reading I can check on it or is it replace it and hope it fixes it? Right now I mainly want to make sure I don't have a major engine problem because I think I am going to be shelling out for a transmission rebuild.

Scav.

BernieB · 10-24-2009, 03:12 PM

Quote:

Originally Posted by scavanger

So then I need to concentrate on fuel mixture or bad coil. I ohmed the wires all looked good.
This would not be a heated O2 sensor , Is there a ohm reading I can check on it or is it replace it and hope it fixes it? Right now I mainly want to make sure I don't have a major engine problem because I think I am going to be shelling out for a transmission rebuild.

Scav.

I'd say take it a step at a time and see what the results are. You've regapped the plugs, let's see if that fixes the problem. If it does, you're home free. If it only partially fixes it, maybe consider a new coil, a good one, not cheap Autozone junk, and check codes again.

scavanger · 10-24-2009, 03:18 PM

What I really need to do is get my laptop fixed. Get a cable and a scanner program so I can do this right. I also have a 90 corvette that I could use it on.Now there's an interesting system to work on.

Scav.

SanshzJ65 · 10-25-2009, 08:22 PM

May be your idle control sensor (goes in the carburator), try removing it and cleaning it with carburator cleaner, it may also be your egr valve. Alternator charging ok?

scavanger · 10-26-2009, 02:56 PM

Quote:

Originally Posted by SanshzJ65

May be your idle control sensor (goes in the carburator), try removing it and cleaning it with carburator cleaner, it may also be your egr valve. Alternator charging ok?

I checked the egr at idle. manually opened it , engine almost died. Unhooked it and drove no change.
The engine does run ok at idle.
Changing to the proper plug gapped helped, I drove it to work this morning and never missed a beat. I have never had .010 larger gap make something run this bad. Also it ran ok for over 10,000 with the plugs that way.Thing is this summer the hotter it was the worse it got, so cooler weather may have something to do with it. I am now pretty sure that with things better with a closer plug gap that it is either a coil or module problem when they get hot. The funny thing is it still starts good hot.Little

on that cause a bad coil usually causes hard start when hot.
Anyway I bring it in on Wed. to have the transmission rebuilt. Will be getting the beast put in and complete rebuild.By By to about $1400

Thanks for the suggestions, good refreshers and some new things for me to keep in mind when I get it back next week.

Scav.

10-24-2009, 12:44 PM	#1
scavanger Senior Member Join Date: May 2007 Posts: 262	95 runs really bad at highway speeds 95 GMC Z71 5.7 auto. 4x4 symptoms: runs pretty good cold. Once it gets to open loop things go bad. At highway speeds it will act like I have a crossfire (maybe not hitting on one causing crossfire?) It will suddenly run good for about 10 seconds or so and then do it again. I have noticed that it will backfire a little.Starts very well, and never dies. I haven't checked fuel pressure yet going to make a fitting to replace fuel filter. (priced on at parts store.$79 ) With long hose I can check going down the road.Plugs,wires,cap, and rotor aren't to old.Also wires not oily,I keep engine compartment clean.The plugs do show rich,, to the point of being black. A few months ago I had number 7 plug wire come off and it ran something like this. Could I have burned through a plug wire ? Bad coil , would it start well cold and hot ? Inside of cap and rotor look good. Right now I am going out to check compression if I can find my gauge, been long time since I used it. Any help would be appreciated. Scav.

10-24-2009, 02:51 PM	#4
BernieB Senior Member Join Date: Jul 2009 Location: Fredericksburg, VA Posts: 1,905	With plugs fouled from a rich mixture, yeah, that .10 could make a difference in the spark. __________________ If the guys who design 'em had to take one home, use it every day for 2 years, and do all the maint and repairs themselves, cars would have a button on the dash to change the oil, filters & plugs.

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10-24-2009, 01:36 PM	#2
Case 7230 Senior Member Join Date: Dec 2007 Posts: 547	My guesses are bad Engine Coolant Temperature sensor, bad Fuel Pressure Regulator, bad oxygen sensor, bad EGR valve or possible restricted exhaust flow. Might get the paperclip out and check for codes?

10-24-2009, 02:45 PM	#3
scavanger Senior Member Join Date: May 2007 Posts: 262	Forgot to mention about 3 weeks ago I got a O2 rich. I did a compression check with my crappy gauge. All over 100 within 10% How sensitive are these ignition systems to plug gap. When I put the plugs in ,15000 miles ago, I must not have been paying attention and gapped them at .045 instead of .035 . I gapped them to .035 ,warmed it up and took it out a couple miles. Was fully warmed up and didn't show any signs of misfire .Not sure if this tells me anything because sometimes it needed to be hotter to effect it. I have never seen anything that sensative to plug gap. Wondering if a slightly weak coil would do this. Scav

10-24-2009, 03:08 PM	#5
scavanger Senior Member Join Date: May 2007 Posts: 262	So then I need to concentrate on fuel mixture or bad coil. I ohmed the wires all looked good. This would not be a heated O2 sensor , Is there a ohm reading I can check on it or is it replace it and hope it fixes it? Right now I mainly want to make sure I don't have a major engine problem because I think I am going to be shelling out for a transmission rebuild. Scav.

10-24-2009, 03:18 PM	#7
scavanger Senior Member Join Date: May 2007 Posts: 262	What I really need to do is get my laptop fixed. Get a cable and a scanner program so I can do this right. I also have a 90 corvette that I could use it on.Now there's an interesting system to work on. Scav.

10-25-2009, 08:22 PM	#8
SanshzJ65 Junior Member Join Date: Oct 2009 Posts: 3	May be your idle control sensor (goes in the carburator), try removing it and cleaning it with carburator cleaner, it may also be your egr valve. Alternator charging ok?