1995 Blazer, 4.3L, 2WD

My engine started missing at idle about 1.5 months ago. At first, it would happen upon restart a few hours after driving it around. Cold start after sitting overnight was fine. Now, I have rough idle pretty much all the time. Ignition coil and ICM are about 8 months old. Just did a tune up; plugs, wires, dist. cap and rotor, air filter, fuel filter. Engine still missing. I've checked for spark using a spark plug tester tool and found there is definitely an intermittent missing no matter what plug/wire I test, but the spark is strong. Did an ohm test, per the GM servicemanual protocol,on the ignition coil and it tested okay.Checked the wire from the distrubor tower to the ignitition coil and it is in good shape. Also checked the wires from the ICM to the ignition coil and they are good. Checkedthe pickupcoil leads, per the GM service manual protocol,with the ohm meter and they appear to be good. The service manual says to change the ICM if the ignition coil checks out okay. Considering itisn't very old, I hate to changethe ICM out already.

Any thoughts from anyone before Igo ahead and change the ICM?

Also, considering I have over220,000 miles on this thing, is there a chance thewhole distributor needs to be changed, and how would I know for sure if I need to do that?

Thanks!

 

Your positive its spark and not vacuum?

If so, sounds like you need to replace the distributor cap and rotor again. I know you said it was only 8 months old. However cheap electrical parts dont last long on these engines. These are high performance engines and need more then just cheap parts. Buy the good stuff, preferrable AC Delco. I spent 3 months chasing a random missfire. It turned out to be cheap spark plug wires and cap and rotor. Cost me $160 for the dealer to tell me that.

As for the ICM, take it out and get it tested before you condem it.

 

Hanr3, Thanks for your input!

Looks like the majority of my missing problem was the ICM. The ICM died today. Went out to start up my Blazer this morning so I could go to work, and it wouldn't start. I went through all the checks on the ignition system and ended up replacing the ICM. Started upwith one try! Though I still have a little rough idle,the idle is much smootherthan what it was before replacing the ICM. I did find a loose electrode post on the "new" distributor capI put in last week, so I put an oldcap back in until I can get a new one tomorrow. Will post again a few days down the road with an update.

 

Cool.
It may run a little rough until your clean teh spark plugs, or they all get dirty. [8D]

You did put the special grease behind the new ICM correct? Heat kills the ICM, and that grease draws the heat away from the ICM and into the heat sink its mounted too.

 

I'll clean the spark plugs when I change out the cap this week.

Yep, I put a nice layer of the special grease behind the ICM.

Thanks again for your input, I do appreciate it!!!

 

your welcome, anytime.

 

Hanr3....You mentioned vacuum, is there any place I can find a diagram of the vacuum lines? I have two GM service manuals of a set of 3 or 4, sadly the vacuum information is in one of the manuals I don't have.

I assume I should be looking for a specific vacuum pressure range, correct?

Thanks.

 

There should be a sticker under the hood with the vacuum diagram, and the psi. Looks for it on the top rad support.

 

try cleaning EGR valve

 

Update...

Okay, after doinga tune up, as stated in previous post, (new plugs, wires, rotor, cap, air filter, fuel filter), then changed out the ICM because the vehicle wouldn't start, I continued to have a rough idle, though it was better initially after changing the ICM. I did clean the EGR valve, it wasn't too bad, and noted its correct operation using my scanner. I replaced all the vacuum lines with new hoses too. I decided to go ahead and change the O2 sensor which caused some improvement in overall engine performance, no change in rough idle however. Also did two tanks of Lucas fuel injector cleaner. Though I do have a slight improvement in mpg after all this, it is still idling rough.

I just completed a fuel pressure test and have come to the conclusion that my CMFI needs to be replaced. The system initially has the correct55-61 psi when the fuel pump kicks on after turning on the ignition switch, but then it bleeds off quite a bit. Per the service manual, I pinched off the fuel line immediately after the fuel pump pressurized the system, and the pressure still dropped. According to the manual, this indicates the problem is a leaky CMFI and it shoudl be replaced. I also took a look in the top of the intake, through the Intake Manifold Tuning Valve holeand saw there is a lot of blackness, especially on the fuel return line side of the plenum. Looks like I will be replacing a CMFI and fuel lines.

Interestingly, about a year ago, I had to have anew head gasket and two new exhaust manifolds put on the engine because two head bolts decided to snap and cause some fun for me. When the engine was apart, I looked at the intake manifold to see if there was any sign of discoloration from fuel leakage, everything looked good. Later in the summer, just for fun, I used a friends bore scope to take a look inside the intake manifold and found there was no sign of discoloration, however it appeared as though there was some slight wetness here and there around the CMFI, although it wasn't like puddles of fuel, just appeared to be alittle damp.There wasn't any rough idle at that time. I'm now thinking thatit is very possible the CMFI or fuel lines began leaking after being disrupted when the engine was torn aparta year ago.

Beforebuying parts, I maytake the intake manifold off to see what is going on. Will have another update soon.

Thanks everyone for your input!