Continuing to battle a Misfire problem 97 Blazer LT 160k Miles.

Only happens at idle. In Parkor neutral, or even drive (slight stall torque) at 1000 rpm or so, misfire disappears. Watching on a scan tool. Always Cylinders 1, 3 or 5 misfiring.

Has new Rotor/Cap/Wires/Plugs. Not all AC Delco, so maybe this is my problem. New coil as well. I have also rotated wires and plugs and stays with the cylinders.

Removed distributor today. Gear is in excellent condition. Didn;t notice anything out of ordinary with the sensor.

Moved onto the spider after reading many threads.

Ran an injector balance test and watched the spray from each nozzle. Looked the same to me. Swapped injectors anyway, on cylinders 1 and 5 with 2 and 6. Misfire stayed with cylinders 1 / 3 and 5.

I am totally stumped. I feel like I must be missing something simple. Could an ignition module cause this type of behavior?

Thanks

 

Have you run a compression test? Or a leak down test? Have you removed the driver side valve cover and had a look at the rockers/pushrods?

 

Has it overheated anytime lately? I agree w/ swartlkk. Do a compression test and/or leakdown test next.

 

i just replaced the spark plugs in my '99 LT. i started with bosch platinum +2's and it misfired like crazy. after replace spark plug wires and determining that everything else was in working condition, i decided to get the AC delco rapidfires (part #12) and it ran perfect.

hope that helps.

 

What about the LIM gaskets? Seems like it could be a problem since they're all on the same side.

 

Thanks for the suggestions. Here is some updated info and also some history of the vehicle as best I know.

Vehicle was from a relative that had been sitting for over a year after having Radiator and water pump changed out, but was acquired with a coolant leak. The vehicle was flatbed 'd and when I started to root cause the leak, noticed it was from the LIM gasket, so I went through the replacement of that gasket, which I believe went very straightforward. No leaks anymore, but maybe there is a better test that I can run.

I did however run a compression test this morning. I don't have the equipment to run a leak down test, but may be that is my next step.

#1 - 140 psi , #3 120 psi, #5 120 psi
#2 - 170 psi #4 160 psi #6 170 psi

Obviously a problem. Could I have messed something up in the LIM gasket repair, or is it more likely that something happened to this engine some time ago that is really causing the problem.

Trying to decide what the next steps should be. Look for a rebuilt engine or try to salvage this one somehow.

 

Have you replaced the intake manifold gaskets yet? if not, that can cause the misfire. its very common for the intake gaskets to cause this. i know someone who changed the gasket 3 times over a couple years and after every time he did it, it fixed his misfire. but now he got rid of the truck because it was to much mantaince for him.
also misfires at idle that clear up when the motor is reved up tell me that the misfire is usually either the basic tune up parts or a mechiancal problem like an inproper valve adjustment or sometime the valve hands up because some carbon particle got between the valve seat and the valve. 2 othe members on this site had a improper valve hanging up and they re-adjusted the valve and it fixed there problem. i have a misfire too on my 1995 but its only at light driving between 1-2k RPM and not under hard acceleration. i also have bosch platuim +2 plugs in there and just read that they cause a misfire....but i will have to confirm that.
you rally need to do a leak down test too. i think autozone or advance auto rent them for free...or harbor freight.com has the tool for 29.99.....

 

Thanks. I'll check out auto zone. Used them for some other tool loans in the past. I did recently change the intake manifold gaket which was in terrible shape. I am concerned maybe I did something wrong there, although it was running terribly before I did the gasket and leaking all over as well.

I also just did a wet compression test. Squirted 3 - 4 shots of oil in each of cylinders 1, 3, and 5. Did not change the reading much. about 3 to 4 psi in each cylinder. Going to remove the valve cover and take a close look.

 

The LIM would not cause low readings on that side of the motor. There definitely looks to be something wrong over there. I would move to a leak down test and see if you are getting any blow by through the valves on that side.

 

Thanks for all the help so far. I couldn't locate a leak down tester today. Will try to find one tomorrow. I did however give the sea foam a shot and interesting enough, after I ran it, cylinders 1 and 3 do not seem to be misfiring any longer. Everything was misfiring at first, then as I drove and cleared the code, 1 and 3 stayed pretty stable. It was a short ride so that may not mean much. I did set a P0305 code and that cylinder misfire was very active. Plan on running it again tomorrow to see what it is doing, then will re-check compression to see what 1,3 and 5 look like after the sea foam and hopefully try the leakdown test.

Since I didn't see much difference with the wet compression check, I would think that would lean towards an issue in the valve train somewhere. I have not pulled off the valve cover yet since I ddn't realize that when I replaced the gaskets, I had the IM removed and all the other hardware out of the way. I didn't see any easy way to get it off without some other major disassembly.

Are there any other tests that I can run to make sure that I don't have some other sort of leak somehwere?

Also, I was looking through some the threads on head gasket replacement and it appears that I could remove the cylinder head with the engine still in the vehicle. Is that true? I have a feeling I am ultimately headed there.

Update: Tried to run a leakdown test (inexperienced at this and had no success). Guage I purchased is either junk ro I don't understand how it works, but I could not get any consistent readings. Not sure of the methodology as I saw some artcicles to leave plugs in, take plugs out, etc.

I did however run an additional compression test after the Sea Foam, and Cylinder 5 was reading 70psi this time and wet checked at about 90 psi. I also checked cylinder 1 and it was at 160 psi this time as opposed to it's earlier reading of 140 psi. Not sure what direction this leads me in. I will try to sort out the leakdown methodology and probably pay a return visit to the store to pick up a new guage.

Any other suggestions would be welcome. Will probably start removing parts to get some access to take the valve cover off.

Thanks