Regular "miss" in idle after UIM/LIM job - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 08-19-2007, 07:16 PM   #1
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Default Regular "miss" in idle after UIM/LIM job

I've been lurking in this forum for the past few weeks as I've undertaken, and just completed, a UIM/LIM job on the wife'* car, a 95 Bonneville SE, K engine, NA with about 187,000 mi on it. She'* an fiercely independent type, so I've always let her handle the maintenance details through her favorite mechanic (GM. Goodwrench auth.). I've monitored her and tried to educate her on how she might be getting screwed over on occasion (I think she'* had way too many transmission and coolant system flushes), but for the most part she'* done a good job of keeping up with repairs, etc. However, since she'* recently lost her job, these duties have fallen to me. And, since I've just purchased a house, I must wrench! It'* been over 25 years since I've done much of that. Enough background...

Looking back I should have paid more attention when she complained of having to add coolant. But she'd just had a water pump done, so I didn't think too much about it, except to keep me up to date. Well, she left for town one Saturday morning and the car just died, she said, on the way into town (2 miles away). She rolled it into a parking lot and called me to say it wouldn't start, it wouldn't even turn over. When I got there, I found it would just start to turn over. I tried to jump it, even though lights looked strong. No effect. I towed her home and layed on the starter until something started to make a whining sound that slowly increased in pitch. I let off quickly.

I found this site and investigated the hydrolock issue. A rachet on the crank bolt would not budge. I removed the front spark plugs and got it to budge about 10 degrees by hand. I took out the rear plugs and #4 (a little hard to turn all the way out) dumped about a cup of coolant. With all plugs out I cranked it successfully, blowing water out of all the cylinders, it seemed from the splash pattern.

Hydrolock, without a doubt. I drained the oil (it was quite low) and put in fresh oil and cranked it a little to work it in. I drained the coolant and put in new plugs (41-606) and gave it a couple of unsuccessful, but very short, cranks. The third one fired it up; idle surged breifly (about 1500 rpm) then dropped to about 900 rpm. I listened for less than 10 seconds for any unusual sounds, then shut it off. It sounded smooth and even, with no knocking or ticking sounds. So...

I bought the APN replacement UIM, and the aluminum LIM gasket set, as well as both radiator hoses, and coolant bypass elbow. I'd never been this deep into an engine before, so it took me longer than you seasoned gearheads, but it was very educational. I took pictures along the way, but nothing you guy haven't seen before. When I removed the UIM there was a lake of coolant sitting on the lower intake manifold; but, to be sure, I probed the UIM in the area of the EGR entry port and found the plastic material to be severely degraded (like thick, wet cardboard) with a small (about 3/16") hole easily discoverable. The LIM gasket looked OK, though.

The throttle body certainly needed cleaning, but I did not do a full disassembly of it. By this time I'm wanting to get this job done. I did spray through the front screen with a MAF cleaner, and I tried to avoid getting any throttle body cleaner past the throttle plate from the other side, though I think I was not entirely successful in that effort as certain areas of the screen turned out to be somewhat golden in color, probably from dirty TB cleaner.

Now, I had purchased the FSM from Helms, so I'd been following it faithfully, and saw no reason not to continue to do so in the refilling of the coolant system. I cycled ignition to on (no crank) a few times to get the fuel system going and used a series of short cranks to start it up. It seemed reluctant to stay running for the first few attempts, then it stayed running (I never touched the gas pedal). I was rewarded with the expected thick white smoke out of the exhaust for about 5 minutes, then all clear. Long story short, it seemed to take a long time for the thermostat to open, but it got to the point where it was open and only coolant flowed from the bleeder.

To the nitty-gritty (finally )--It seems like it'* missing at idle, except that that it'* extremely regular and at too slow a rate to be a regular miss. When I give it a little gas it'* a lot smoother, then rpm drops back to just under 1000, like it'* supposed to, but right them it takes a brief dive, like it wants to die, but it catches itself back up for between 1 and 2 seconds, then dives again. It will do this over and over again like clockwork. I put it into reverse (no gas) to roll back a few feet so I could clean up the floor underneath, and it didn't die, but the pulsing was quite evident--the same when I pulled back forward.

I've spent hours over the past few weeks reading messages in this forum, and have caught some talk of O2 and MAF sensors and the IAC valve, but nothing that seems to address my issue in particular. Can anyone help me understand what is likely happening in my particular case? Thanks in advance to anyone who has read this entire message--and I'll put my second-born in the mail to anyone who can help me solve this problem.
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Old 08-19-2007, 07:36 PM   #2
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How old is the 2nd born? I have 2 teens & don't need another
Try unplugging the MAF sensor & see what happens.
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Old 08-19-2007, 07:43 PM   #3
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Did you say you didn't remove your MAF while cleaning the TB? Any chance the sensor was damaged?
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Old 08-19-2007, 07:46 PM   #4
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Quote:
Originally Posted by Technical Ted
How old is the 2nd born? I have 2 teens & don't need another
Try unplugging the MAF sensor & see what happens.
I was playing it safe--there is no second-born, and I don't expect one. But if the unexpected happens, it'* yours.

I'll try unplugging the MAF. Thanks.
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Old 08-19-2007, 07:50 PM   #5
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Quote:
Originally Posted by dillcc
Did you say you didn't remove your MAF while cleaning the TB? Any chance the sensor was damaged?
I don't know...Could it be damaged by TB cleaner? Or by MAF cleaner? I was careful to not knock the TB around when I was handling it. Would the MAF be sensitive to vibratory shock? Can I detect the kind of damage you are talking about visually? Thanks.
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Old 08-19-2007, 09:02 PM   #6
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Quote:
Originally Posted by Technical Ted
How old is the 2nd born? I have 2 teens & don't need another
Try unplugging the MAF sensor & see what happens.
Well, Ted, that was dramatic! It had been a couple hours since I shut it down, so it was pretty cold. It died after the first three starts, so I gave it some gas. RPMs kept rising for a second after I took my foot off the gas, then settled to about 1000, and steady, with no "missing". The "Check Engine" light went off, too. I put it into reverse gear and it was still smooth. I put it back in park and the idle stayed at about 700 rpm, where it was while in gear. I gave it a little gas and it bounced up and settled at about 1000 rpm. As it warmed up it suddenly started "missing" again (Check Engine back on), especially badly, it seemed, when the heater was on full. It would drop to about 600 rpm, and the interior lights would dim until it revved itself back up. These "misses" are less regular than I'd thought before, and the rate is more like a half second. By now the thermostat is open (gage reads just under the 200 mark) and it'* "missing" badly as it was earlier. I tapped lightly on the MAF with a screw driver handle--no effect. Then I disconnected the MAF connector. The "missing" got more pronounced, that is, engine speed varied more and more widely, going lower and higher each time, until on about the third time I thought it was going to stall--but, instead, it steadied right out! No more "missing", or pulsing at all! Smooth in gear, too (Check Engine still on). I left it that way for a minute or two, then plugged the MAF back in and immediately it was up to its old tricks again.

I'd bet when I unplugged the MAF the computer finally failsofted the MAF'* contribution to its calculations, and so engine speed was steadied. Does this mean I definitely need a new MAF? I had found some coolant in the air duct when I started this project. Is the MAF sensitive to coolant? Can I just clean it somehow? They appear to be a tad pricey.

There'* no smoke, or water coming from the exhaust pipe at any time, but when I disconnected the MAF there was a subtle change in the odor that I can't describe.

So, there'* the answer to your question, Ted. Now, what does it all mean?
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Old 08-19-2007, 10:02 PM   #7
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It sure sounds like the MAF is giving you trouble. CRC makes a mass air flow sensor cleaner that you just spray on the sensor. Some here use a cotton swab & alcohol to gently wipe the sensor clean. The sensor is very pricey & I hate to recommend replacing it without getting a scan . Unfortunately its hard to find a scanner for the 94 & 95'*.
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Old 08-19-2007, 11:07 PM   #8
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Quote:
Originally Posted by Technical Ted
It sure sounds like the MAF is giving you trouble. CRC makes a mass air flow sensor cleaner that you just spray on the sensor. Some here use a cotton swab & alcohol to gently wipe the sensor clean. The sensor is very pricey & I hate to recommend replacing it without getting a scan . Unfortunately its hard to find a scanner for the 94 & 95'*.
So you don't think coolant could have destroyed it, just contaminated it? I'll try to clean it and see what happens. Thanks for your help, Ted.
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Old 08-20-2007, 09:52 AM   #9
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I'm not sure it'* the MAF. It appears from your description that unplugging is defaulting you to higher rpms and smoothing the problem.

How old would you say the plug wires are? All are on and hooked up tightly? The #6 isn't touching the O2 wiring correct? Also.. have you pulled and checked all the plugs. It'* possible one is causing an issue.

My other thought was ...vacuum. Did you get all the vacuum connections back on and none are leaking? The U bend at the TB is a very common place for the line to split and cause issues.
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Old 08-20-2007, 10:45 AM   #10
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Sorry to sound like a broken record in this forum, but check all around for vacuum leaks.
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