Intermittent miss at light load.

534BC · #1 08-17-2006, 10:15 AM

My Blazer 95 W code has had an intermittent mis-fire only at part throttle for a long time. It is getting worse now, any ideas on how to check it out?

rmacd930 · #2 08-17-2006, 01:38 PM

I would suggest checking your plugs and wires, the 95s wires run really close to the exhaust manifolds and they tend to dry up and crack easily. If they have never been replaced then they will probably come out in pieces...

534BC · #3 08-17-2006, 05:09 PM

Plugs, wires, cap and rotor are all new about a few months ago. The problem is still there though. It is so intermittent it is hard to make it act up when I want it to.

All I have observed indicates secondary ignition components. I am pretty sure that it is worse when it is raining with the lights on.

534BC · #4 08-17-2006, 05:12 PM

Some more info.

It is not consistant, usually it runs great.
It has been there about 2 years I estimate.
No codes , no light
Never happens at WOT
Never happens at 1/2 throttle or more
Never happens above 80 mph
Never happens at idle

BlazerLT · #5 08-17-2006, 09:54 PM

CPI injector and nutkit are probably leaking.

534BC · #6 08-17-2006, 10:00 PM

Hmm, I was thinking spark, never thought of fuel problem, I did raplace a small plastic line under the intake a few years ago, it was leaking gas into the intake. I think I know how to check it.

Pressure up fuel system with key on and then turn key off and wait for pressure to drop or hold? Maybe could be a leak AFTER this pressure test, but why would a leak or rich mix on a cyclinder or two cause an intermittent misfire?

How else can I verify this without a lot of disassembly?

BlazerLT · #7 08-17-2006, 10:25 PM

Remove the top vortec cover and then remove the round IMTV valve and peer down into the plenum and looked for washed spots.

Also smell for gas and look for pools of gas in the plenum.

534BC · #8 08-19-2006, 03:16 PM

I think it is not fuel problem, it is a misfire. I had it in my head that it was worse when wet and/or night out with the lights all on.

Then I thiought it was worse when the engine was cold. Today It was rainy, very humid and so I started it up, turned on the lights and ac fans and took off imediately, It didn't act up until about 10 minutes into the drive.

So much for bith of those theories, lol. bummer very inconsistant problem.

534BC · #9 08-19-2006, 03:19 PM

Worst case scenario is at constant to very slight acceleration between 1000-3000 rpm and between 30-55 mph. It must be used to those parameters, that's what I usually run .

I hope it dies soon so I can figure it out.

BlazerLT · #10 08-19-2006, 03:44 PM

Quote:

Originally Posted by 534BC

I think it is not fuel problem, it is a misfire. I had it in my head that it was worse when wet and/or night out with the lights all on.

Then I thiought it was worse when the engine was cold. Today It was rainy, very humid and so I started it up, turned on the lights and ac fans and took off imediately, It didn't act up until about 10 minutes into the drive.

So much for bith of those theories, lol. bummer very inconsistant problem.

Don't ever assume it is not one thing over the other.

The CPI injector leaking can and will cause a misfire seeing it cannot hold the fuel pressure needed to open the poppets and limit the fuel going into the cylinder.

It could also be a coil that is gone.

534BC · #11 08-19-2006, 03:47 PM

Yes, thanks. The coil is a real good bet, I may become a "parts changer" and replace it if it isn't too expensive, I do not know how to check it other than to inspect it very well or replace. I can't dump a lot of money into this right now.

534BC · #12 09-19-2006, 08:23 PM

It finally got so bad and after watching it in the dark I decided to buy a coil and put it on. I seen a nice little light show on both edges of coil by the metal bracket so before I got a new one I insulated the coil from any metal mountings and cleaned the sides up real good.

A few days later I concluded it was "fixed" almost so I got a coil and replaced it. It was "fixed" for a few days (with 2 different drivers that are aware of problem)

A few days later I conclude that it is 50-90% better, but is not fixed at all. Ran new coil enough to conclude that it is still misfiring very intermittently, but not as bad.

A few days later I put old coil back on and ran for a week or so concluding it is worse than the new one. I also have drowned it in enough water to make an off-roader jealous and when it dries off it runs exactly the same.

534BC · #13 09-19-2006, 08:28 PM

After putting old coil back on it runs bad when warming up, but when warm seems to run ok and not miss. The other nite I saw another little light show at the sides of the coil and also found some intermittnt sparks jumping to my dipstick tube and also the coil wire was jumping to the metal braket.

Ho, humm, insulated the tube and some wires that were close to metal and moved coil wire away from metal braket, time to test drive.

BlazerLT · #14 09-19-2006, 11:27 PM

Get the new coil on there, the old one is pretty bad if it is arching like that.

muddog321 · #15 09-20-2006, 05:16 AM

I had replaced my cap/rotor with Wells and in 1 year it was arcing inside the palstic casting between wires (not visible even looking at the inside) so just a point to consider - many aftermarket parts are not equal as I chased and traced this for a few days - a simple cap/rotor change cured and only way to tell for sure. If all is OK you replaced already then look at the crank sensor ($60) and ign module (in dist on yours $75).

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