Ok long story. I had the pickup coil on the distributor go out. In the course of this discovery I had the plug wires, dist. cap, rotor, fuel filter,spark plugs replaced 2xand eventually the distributor itself replaced. On a side note I replaced the plugs a second time because the bosch platinum plugs the mechanic installed the last timewere horrible and I had to get the truck through emissions.All this and myengine nowhas a horrible missing/bucking whenI am cruising at any speed. When the throttle is wide open no missing no bucking. WhenI let off the gas completely no missing no bucking, but if just keep my foot on the gas to maintain my speed the whole motor shakesitself nearly to pieces.I had the timing set at a shop with a timing light and whenit was setthe motorwould hesitatewhenyou gave it gas and it would pop and miss and highway speeds or if you laid on thegas. I have it a bit more advanced now and it accelerates nice and no popping but it misses and bucks at cruising speeds. Please any and all help would be greatly appreciated. I love myK5 butwith all of the recent trouble and expenses my wife is trying to make me get rid of him.

 

This sounds like a throttle position sensor problem... I would test the voltage coming out of the TPS for dead spots during the opening/closing of the throttle. It is typical to have a dead spot where the throttle is normally positioned during cruising as the frequent minute changes in throttle angle while driving will actuall wear away at the sensor grid, resulting in a dead spot. The TPS should operate between 0.5V (closed throttle) and 4.5V (wide open throttle) and be a smooth transition between the two.

Please include your vehicle information in your posts so we can be more accurate with our diagnosis. From looking at your other posts, I see that you have an '89 K5.

Hope this helps!

 

What I have is a 1989 full sizeGMC Jimmywith the V8-350 (5.7L). I will check the TPS this weekend. I appreciate the help and am open to all comments, suggestions, and possibilities. There is one other thing I know. For the last 4years that i owned thejimmy i have always got a code 32 EGR after running the truck for about 10 mins at highway speed.I have never noticed an adverse affect on performance when the light came on. Ibought a new egr valve and it was a pos oem so I ended up returning it,cleaning the origional and it seems to be functioning as it is intended. I have checked or replaced the egr solenoid and map sensor as well. If i can offer up any more info please let me know.

 

*Update* SoI replaced the plug wires again thinking I had a spark problem didn't seemd to help.I checked the TPS and that wasn't it. I then tweaked the dizzy a little bit more and got it to the point where the truck only missed rarely at crusing speeds. At this point it would happen only a couple time per hour drive instead of every time I let back on the throttle. So i broke down and took it to a shop. I thought maybe they could at leastdiagnose my problem if notfix it. This is what I have been told. I just want to run this by everyone here because this could spell an end to my truck[:@]. They did a compression test and said they found that my #4 cylinder was at 45 psi. They also said they checked with an oscilloscope and determined it was not the valves but the piston rings. Then they tell me that I need to replace the engine with a new one to the tune of 5grand. 1Idon't think a crate motor even with labor should come to 5 grand. 2.Is this the way to check if its the valves and not the piston rings (have heard of the wet test). 3. Is this the only thing it could be? My truck isn't backfiring/burning/ or doing anything irregular other than the random skip/miss at cruising speeds. Any and all thoughts or comments are greatly appreciated.

 

5 grand is that with them doing the eng plus labor?? You can get an eng off ebay "new" for 1700.00 do it yourself and save on the crazy labor..

 

Squirt a bit of oil into the suspect cylinder, turn the motor over with the starter a bit to distribute the oil, then redo the compression test. A compression tester can be had for around $30 and is a good tool to have in your toolbox.

If it is the rings (mostlikely not the piston), then you could do a quick rebuild and be on your way, but a crate motor would be a better diy job for a backyarder.

 

I am kinda curious how an oscilliscope can tell the difference between bad rings, and bad valves.... Seems to me, that is a line of bull feces. (Anyone know otherwise? I have never heard of this before, and have been using O scopes for quite a while....) Go to Autozone/Advance Auto, rent/borrow/buy/steal a cylinder Leakdown tester. This will require having a source of compressed air.... the leak down tester will tell you for SURE whether it is rings, or valves. (just listen to where the air is coming out..... exhaust, or intake, valves. PCV port, or, oil fill cap, rings.)

Or, take it to another shop, do NOT tell them the previous shops diagnosis, and see what they come up with.

 

SO compression is fine. I ran a couple cans of seafoam throughc the pcv and gas tank. The engine runs real strong and with it running as strong as it is, combined with the fact it burns no oil, blows no smoke, doesn't backfire, idles, accelerates, and drives at on all 8 cyl I am now convinced it has to bean egr issue. Here is what I got. Everything has really evened out. I am left w the same old problem, but now it's really obviousIonly get my stumble/miss at cruise 50mph+ when the engine has warmed up to full operating temperature. When cold = no stumble 50 mph+ andthe stumblehappens in unison with full op temp &the ses light. Code 32 is the everpresent factor. I am going to clean and inspect the egr valve again, put on a new gasket, new hoses,clean out theegr solennoid andsee what I get.If anyone has a suggestion to add while I've got this stuff apart for cleaning let me know.