Rough Idle - Missing
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Rough Idle - Missing - 3/31/2008 8:09:25 PM
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cfurg
Posts: 5
Joined: 3/31/2008
Status: offline
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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 service manual 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. Checked the pickup coil 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 it isn't very old, I hate to change the ICM out already.
Any thoughts from anyone before I go ahead and change the ICM?
Also, considering I have over 220,000 miles on this thing, is there a chance the whole distributor needs to be changed, and how would I know for sure if I need to do that?
Thanks!
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RE: Rough Idle - Missing - 3/31/2008 9:52:18 PM
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Hanr3
Wheels & Tire Moderator
Posts: 3523
Joined: 5/11/2005
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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.
_____________________________
"Some people spend an entire lifetime wondering if they made a difference.
The Marines don't have that problem."
-President Ronald Reagan
'87 S10-Blazer - SOLD
'93 S10-Blazer 4x4x4 - Sons DD, engine swap is done.
'00 S10-Blazer 4x4x4 - My
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RE: Rough Idle - Missing - 4/1/2008 6:52:34 PM
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cfurg
Posts: 5
Joined: 3/31/2008
Status: offline
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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 up with one try! Though I still have a little rough idle, the idle is much smoother than what it was before replacing the ICM. I did find a loose electrode post on the "new" distributor cap I put in last week, so I put an old cap back in until I can get a new one tomorrow. Will post again a few days down the road with an update.
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RE: Rough Idle - Missing - 4/1/2008 9:28:58 PM
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Hanr3
Wheels & Tire Moderator
Posts: 3523
Joined: 5/11/2005
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Cool.
It may run a little rough until your clean teh spark plugs, or they all get dirty. 
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.
_____________________________
"Some people spend an entire lifetime wondering if they made a difference.
The Marines don't have that problem."
-President Ronald Reagan
'87 S10-Blazer - SOLD
'93 S10-Blazer 4x4x4 - Sons DD, engine swap is done.
'00 S10-Blazer 4x4x4 - My
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RE: Rough Idle - Missing - 4/2/2008 1:33:03 PM
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Hanr3
Wheels & Tire Moderator
Posts: 3523
Joined: 5/11/2005
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your welcome, anytime.
_____________________________
"Some people spend an entire lifetime wondering if they made a difference.
The Marines don't have that problem."
-President Ronald Reagan
'87 S10-Blazer - SOLD
'93 S10-Blazer 4x4x4 - Sons DD, engine swap is done.
'00 S10-Blazer 4x4x4 - My
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RE: Rough Idle - Missing - 4/2/2008 3:05:42 PM
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cfurg
Posts: 5
Joined: 3/31/2008
Status: offline
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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.
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RE: Rough Idle - Missing - 4/3/2008 8:16:31 PM
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Hanr3
Wheels & Tire Moderator
Posts: 3523
Joined: 5/11/2005
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There should be a sticker under the hood with the vacuum diagram, and the psi. Looks for it on the top rad support.
_____________________________
"Some people spend an entire lifetime wondering if they made a difference.
The Marines don't have that problem."
-President Ronald Reagan
'87 S10-Blazer - SOLD
'93 S10-Blazer 4x4x4 - Sons DD, engine swap is done.
'00 S10-Blazer 4x4x4 - My
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RE: Rough Idle - Missing - 4/26/2008 5:28:57 PM
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gscrippler
Posts: 78
Joined: 4/26/2008
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try cleaning EGR valve
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RE: Rough Idle - Missing - 4/29/2008 6:34:10 PM
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cfurg
Posts: 5
Joined: 3/31/2008
Status: offline
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Update...
Okay, after doing a 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 correct 55-61 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 hole and 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 a new 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 a little damp. There wasn't any rough idle at that time. I'm now thinking that it is very possible the CMFI or fuel lines began leaking after being disrupted when the engine was torn apart a year ago.
Before buying parts, I may take the intake manifold off to see what is going on. Will have another update soon.
Thanks everyone for your input!
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Same problem here..... - 4/29/2008 7:15:19 PM
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nimrodcs
Posts: 8
Joined: 4/20/2008
Status: offline
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My 2001 Blazer has the same rough idle and a missfire indicated on #2 cylinder. Thinking it was an ignition problem I changed the plugs and wires, distributor cap and rotor, checked the ICM (it was fine), fuel filter, air filter, fuel injector for the #2 cyld, and the intake manifold gaskets while I was at it. It ran a little smoother, but still idled rough and had the missfire code. I finally traced it to a stuck exhaust valve. I have no compression in the #2 cylinder what so ever.
I can hold a piece of paper under the tailpipe and it will suck it up and then spit it back out. Thus, I'm pretty sure, is a valve problem. This might not be your problem, but it might be worth it to check it out. Hope this helps man, good luck.
-nimrodcs
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RE: Same problem here..... - 4/30/2008 4:00:13 AM
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cvcmama22
Posts: 10
Joined: 3/12/2008
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not saying this is your problem either but did u check the valve springs mine jimmy was ding the same thing i put all new stuff in it and still didn't fix the problem came to find out the valve spring a $2 item was the problem with that many miles the springs r probably getting weak if they r not already
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RE: Rough Idle - Missing - 4/30/2008 10:31:41 AM
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Duntov
gone
Posts: 224
Joined: 2/29/2008
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I was pleasantly suprised when I found a engine misfire with my '91 Blazer 4x4 . It began a slight miss not long after my daughter had the battery replaced. The miss was only noticeable in top gear of the 4-speed automatic, especially on a slight grade.
It turned out ot be slightly loose battery cables. I took 1/2 turn on the side terminal bolts and later test drove the SUV and the miss was completely gone. I would have never thought that would cause a intermitten engine miss because I am used to 50s and 60s cars that can run fine without battery cables after you get them started.
Could loose battery cables have been the cause of the engine miss or was loose cables just a coincedence?
_____________________________
1991 S10 Blazer 4-dr, auto., 4x4, 4.3L TBI
Competely original. My Blazer even has 1991 air in the spare.
Restored 1963 Pontiac Catalina Sport Coupe, Richmond ST-10 4-speed, 421 H.O., 370 HP tri-power, 3.64:1 Saf-T-Track, HD Police Option
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