Shutter/Miss at highway speed
 

Hey guys, I have another one for ya. I have a 2000 Silverado Step Side 4.3L. The problem I'm having is that at highway speeds around 70-85 I seem to be getting a random jolt, almost seems like the engine just misfires randomly. The truck will be running smooth and then just hit a snag and then immediately go back to running smoothly. I can feel the jolt from the engine every time it happens.

To give you a bit of history on the truck, I tuned it up with new plugs, wires, cap and rotor, pcv, fuel filter, and removed the stock air box and put a K&N intake filter (not the airbox drop-in) on the stock intake pipe, and removed the resonator box and capped it. I seafoamed the truck a few months ago through the brake booster vacuum line, and also sprayed and cleaned the Throttle body as best I could without removing it.

The truck picked up alot of power, as it was bogging/droning from the carbon. The truck had a surpising amount of carbon, which we noticed when we replaced the gasket for the ERG valve.

The truck is not throwing any codes at all, and random skip/miss seems to only happen at about 70-85 (I don't drive at higher speeds in the truck).

Any ideas on this one?

The random issues are always a pain to diagnose. Is there anything in common with the times it misses besides speed. Heat, distance traveled, rain, etc. Or does it do it every time you are traveling that speed? When you replaced the EGR valve gasket did you test or clean the valve itself?

EGR Valve
 

We did run some cleaner through the EGR valve. I don't seem to be having any isues with heat. The distance doesn't really seem to matter, weather doesn't seem to matter either.

Also seems like the truck starts up great sometimes and other times it seems like its sluggish to start up. Not sure what to make of it.

Also, I didn't test the EGR how would we test that?

Egr good if it was not performing right would get a cel. I guess no service light?
Did it start after k&n filter was installed? Could be causing maf sensor to malfunction.
If not getting service light .. maybe problem. Other cause could be worn dist. gear.

I had a similar issue on my 94 Ford 5.8L. Turned out to be a weak valve spring.
The engine as 175,000 miles on it. I was able to pump the cylinder up with compressed air, and change the bad spring without removing the head.


Should probably do a valve job on it, and replace all of the springs, it's just not a priority, right now.

Starting to think
 

Ok, so I'm starting to think the issue is with rain getting onto/into the K&N. It's been raining on and off here in CT. Today it downpoured heavily and I noted that the car was skipping again primarily at takeoff. So when I got to work this morning I popped the hood, and checked the air filter. Sure enough there was water on it. It wasn't saturated but there was water on it. When it was installed the black metal plate under the stock air box was removed to get more air directed towards the filter. It appears that it's also directing rain water in its direction as well.

I plan on reinstalling the plate later today when I get home and getting some water remover from Autozone today to run through the fuel lines to see if this helps the situation. Anything else I should be doing? Changing the oil?

No check engine light at all, but the skip is definately still there.

I wouldn't worry about the fuel stuff, but an oil change might not hurt.

Might pop out a spark plug and look there too.

You can tell alot by looking at spark plugs and even O2 sensors.

So I changed the oil in the truck today, and sure enough there was water in it. The truck ran much better tonight, and I will continue to monitor it during the week. I found a random misfire code stored in the ECM and cleared it.

Not sure what exactly to check on the spark plugs, so I didn't pull them.

What should I be looking for?