Rough Running Engine

I have a 1993 Chevy Silverado regular cab that is lifted 9 inches, has 435 front and rear end, a 5 speed manual, 38inch tires, and a crate 350 4bolt main with maybe 50,000miles.

Keep in mind that it has been sluggish for 4ish months and I have replaced Plugs, wires, cap, router bug. But it has been sluggish and not quick to get up and go like it should when you mash the gas pedal. It's kinda delayed.

Anyways, I was driving down the interstate and I down shifted about 70-75 mph from 5th to 4th and I did so to get some rpm's and speed to get around someone and my tach never went about 4000rpms. But it started to kinda tick and maybe have a little bit of a knock. Well when I got to my destination it was running pretty rough and it definitely shook everything inside the truck at idle from how rough the engine was running. I managed to make it to my apartment and never lost oil psi and it never began to smoke. I've done some research and it almost sounds like a Knock Sensor went bad cause of the previous sluggishness. But I wanna know what someone else thinks. Please.

Ok, um two codes came up: one pointed to the "environmental regulator", saying it was not working (we had to bypass it when we put in the propane system) and the other code came up as the "fuel regulator" which we had checked and cleaned on both the propane and gasoline systems.

Side note: my father recommended checking the silicon compound on the ignition module. The idea being if the module got too hot and there was little/no silicon, the lack of heat distribution would burn up the part and shut down the engine. Or the computer would read the problem, and kill the engine. When I checked the part, it does indeed lack the silicon on the metal plate. It had dried out and flaked off a long time ago.

Unsure if there is an EGR problem, with the EGR valve disconnected there is no change and the valve when hand tested operates like a new one. The power brake hose is not pinched or obstructed if that is what you mean. I did try to depress the brake pedal to put the engine under some load when it was running and there was no change in operation or unusual change in vacuum. The engine needed the crank ground and also nad a worn cam lobe, so I replaced the cam with a stock cam, and replaced the lifters, just for more info.

I'm embarrassed to say this, but when I crawled under it to change the Knock Sensor, I spotted a spark plug wire that popped off! It runs like new again! Thanks for the help.

I am a diesel mechanic and have been wrenching for 17 years. The machining work and cleaning was done at my diesel shop under my supervision so I am sure it is accurate and everything is clean and in good condition. There was a gap in the intake manifold after manifold was set in place although I used extra sealant on the front and rear of the mating surface. When I realized there was a gap I sealed it by pressing the sealant into the gap and added extra on the outside, but I am thinking if there was a problen here the engine would not produce normal vacuum initially.

Ok, thank you! I changed the part last night and test drove the truck this morning. It seems to be doing much better! And no light. I really appreciate your quick responce, thank you very much!

When the PCV is blocked off the idle increases a little but the vacuum problem is still present. I blocked the PCV for a few attempts and got the same results, however when the PCV is disconnected the engine will start better and sometimes did not need the throttle opened to start, and on the first attempt idled on its own at around 300 RPM for almost a minute.

I set the valves at 0 lash plus 3/4, then re checked them while I had the engine running.

I have a 1995 Silverado - 2500 (3/4 ton) 4x4 with a 454 (7.4 liter) engine and I just replaced the (automatic) transmission this spring. Since I've gotten the truck back with the new transmission, it has been running terrible - RPMs all over the place, 'coughing', sometimes backfiring... And it dies or stalls almost every time I'm at a stop light or when the wheels stop moving - I need to use two feet (brake & gas) to keep it running at a light.

The 'check engine' light has been on (but it has been since I bough the car [used] a year and a half ago after my previous Silverado was totaled when I was rear-ended by an SUV - but that's another story.)... The codes I've checked since trying to diagnose this running problem indicated the EGR valve and MAP sensor - both of which I've replaced - and neither of which has solved the problem. We also checked the coolant temperature sensor - unplugging/bypassing it seemed to solve the running/idling problem - replaced this [relatively inexpensive part], but again, no fix. I've also replaced the fuel filter and ignition module (more recently it's been giving an 'ignition system' code) - neither of which have solved the problem either.

I've tried all of these:

EGR valve
MAP sensor
coolant temperature sensor
fuel filter
ignition module
premium/hi-octane gas

none of them have solved my problem.

What can be wrong with my truck? Might I have been the victim of sabotage? (Strangely, not long after the running problem started, each of the four tires needed to be replaced inside of a ten day period - each from seemingly unrelated events - leaks, nail/screw, etc.)

Any insight would be of great value.

THANK YOU.

Hi Joe -

Thanks for your prompt reply to my query - -

I really don't know if someone might've put something in my fuel tank - but I suppose it is possible. I also suppose I'd have to remove the tank and actually look inside it to see if this is the case? I believe the mechanics helping me (who're both perplexed) checked vacuum pressure - but I don't know if that rules out leaks or not. I will check again and get you the most recent codes - the last codes I know were for the ignition system - which is why we replaced the module. I have not tried fuel injector cleaner in the gas, but I believe my mechanic checked and cleaned the injectors. I replaced the EGR myself and I did not notice any carbon build-up on either the intake or vacuum hose - apart from a very tiny bit of grime (which I scraped off with a razor blade before installing the new gasket and then the new part.)

Thank you for your questions! I hope this info helps clarify some issues - - I'll check the codes again and post the results ASAP.

- PR

Hello again! I have had some other issues taking unfortunate precedent for a bit (clutch/t.O. Bearing and more for my other truck - a Chevy '70' dump truck - and minor repairs/maintenance on a New Holland skidsteer... AND, believe it, other trouble with, I think, the starter for this truck now too! Seems to draw down the voltage from 13V to less than 9V every time I start it - - just this week needed to jump it several times to get going. Test shows the battery to be good. Could this be at all related to my other issues?)

But back to my still persistent running problem(s)...

The codes I now get are "36" (Idle Speed Control Actuator - which sounds very likely to me) and "54" (Fuel Pump Relay - sounds likely as well... But not as sure how to proceed with checking this one myself though). I also get a "12" code, but I think this just indicates the diagnostic feature is working.

Thank you!

-PR