Okay, Here's a Weird One

1990 Chevy Suburban, 5.7L, 700R4 trans., TBI, 178,000 miles. It has been surging (especially under load) and I thought that I had fixed the problem a week ago but now that it's cold, the problem has re-appeared.
I did a tune-up aout a month before the problem started (plugs, cap, rotor). Fuel filter was replaced about 5,000 miles ago and EGR valve and control solenoid are both new The problem originally began immediately after a 3-hour ferry ride on rough water (and the air temp dropped significantly too).
Now for the weird part. The engine idles rough and slow before it is warmed up but after it's warmed up, it varies from normal/smooth to rough/slow. If I hook up my Sun Scantool, idle speed increases (it happens as soon as I tell it to read codes or to read the data list). The idle goes to about 1000 RPM in park and if I turn the power off to the tool, the idle decreases until it almost stalls (you can power up or power down numerous times with the same results). Could it be a faulty ECM? I've hooked the tool up to other vehicles and it doesn't affect the vehicle's operation like it does mine.
Also, last night and today, I've been hearing beeps coming from the direction of the ECM. They are very high in pitch and very low in volume. I've never heard these before and it only started after my problem began yesterday. Does the ECM emit any kind of audible signal?
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The ECM has no speaker of any type and no moving parts to make any noise either, so it's probably not the ECM making the racket. I'd check the torque on your TB-to-intake bolts as they have the tendency to come loose after awhile and cause a small vacuum leak.
I would also check the connections to your CTS and see if they've corroded out or worked their way loose over the years.
Doc

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The ignition wires look good and test good but they've been on a long time so I may replace them anyway. Doc, I'll check the torque on the intake manifold bolts and I'll check the CTS connections for any problems. This problem is so intermittent that I can drive for a week or more with no problems and then - it hits. I didn't think that the ECM should be making noise but it came from that direction and it was in a particular pattern so I thought I'd ask (besides, the residents of the planet Hurkon aren't supposd to contact me in my truck LOL).
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you may also want to look into the cam shaft sensor

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I wasn't aware that this engine had a camshaft sensor. If it has one, where would it be located?
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On Fri, 19 Dec 2003 22:05:16 -0500 (EST), snipped-for-privacy@webtv.net (Rich B) wrote:

TBI's don't have a cam sensor
-Bret.
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Does the noise you mention only happen when you turn the key to run, and when you turn the key off? If so, it's the HVAC recirc door opening and closing. My 88' does the same thing........................sounds like it's taking a dump................
Doc

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Doc, it only made the noise with the engine running and I only heard it for a few days. It was three quick, high-pitched beeps. It almost sounded ike an alarm on a wristwatch but I don't wear a watch and there was nothing in the truck that would makes a sound like that. I really doubt that the noise has anything to do with my problems but I thought I'd ask.
Sometimes, the engine will bog down and have no power and if you floor it sometimes (but not always) it will accelerate normally. One time (and only one time) it began to ping and knock when I was going up a steep hill. I originally thought that it may be the ESC module or knock sensor malfunctioning, retarding the timing (i think that it can retard the timing by as much as 20 degrees) but I'm not sure how to check these two items.
Right now, the truck is running normally again and I have no way of knwing when it will malfunction again. Occasionally, even when it's running good, I'll get a "flat spot" when I try to accelerate slowly. Also, I've noticed that when I'm sitting at idle for too long, the idle begins to drop and if I sit too long, it almost stalls (this just started to happen).
A year ago, the dealer replaced the TBI throttle plate because they said that the shaft was worn, knocking the TPS out of wack and causing some of the problems that I'm having now. When they changed the TBI throttle plate, the problems went away until November so I doubt that the throttle plate wasn't bad. This truck was a Surveyor's vehicle before I bought it so its been used pretty hard (and not maintained very well mechanically).
I haven't gotten much time to work on it the last few days (and besides, it's cold as a witch's tit), so I haven't been able to eliminate anything as a cause. I thought I'd put it out to the group to see what you can come up with (a bit of brain-storming if you will).
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snipped-for-privacy@webtv.net (Rich B) wrote in message

Did you replace the spark plug wires with your tune up as well? That and an old, weak ignition coil was what was causing my Jimmy to stall in my previous posts. Wires are cheap. Damp weather will cause any minute cracks/leaks in the wires to start grounding on surrounding metal, which will cause a stall/rough idle.
-- orko
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Doc, I've been suspicious of the KN sensor but haven't checked it out yet. I will be checking th wiring as soon as the truck cools down and I thought that the O2 sensor had only a few thousand milesbut it turns out hat it has 36,000 miles on it so I guess I'll replace it (I usually repace them at 30,000 miles). I did find out that the ESC on the engine was the wrong part. I checked the P/N and it turned out to be for a 1987 - 1988 6-cylinder or a 1987 Caballero, El Camino 4.3L only. I called a friend that used to be a GM mechanic and he said that it might make a difference so I bought the correct one and guess what! - no noticeable difference (oh well, it was worth a try). I appreciate all the help that I've gotten and when I find the answer to this problem, I'll let you know what it is (but I still think that it might be space aliens LOL).
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Just a shot in the dark but on my 92 I also had a similar problem.
Bogged down when departing the stop light but only intermittently, Didn't seem to have the power it should. Also had to crank longer to get it started although not really noticeable until after I got it fixed. Then one day it decided today was the day and really began to run like shit until I got it 25 miles home to my mechanic, then it ran great at which point I gave it to him for the weekend to drive. When it got really bad holding the accelerator to the floor would redline the rig but it would not shift until I let up a little, I though I had some funny transmission trouble. Turned out to be a bad injector. I had checked them several times before, but evidently it was running good at the time. Anyway after the new injector it ran like new.
Just a thought. Good Luck
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Hmmm, I have thought about the injectors but I haven't checked them yet. I haven't gotten any codes so I don't really have anything to point me in a particular direction. Free time is a scarce commodity right now and add the cold, wet weather to that and you know why I haven't found the problem yet - but I will eventually. I don't have a salamander heater or anything like it and if I did, I wouldn't have a place to store it but I figure I can drive the truck into an abandoned garage (we have some around here) and then torch the garage to keep warm (just got to remember to get out before it kills me LOL).
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Did you reset the ECM after you replaced the ESC? If not, do it.
Doc

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Bang on your Cat converter. If you hear marbles or mice running around or cats scratching to get out, I'd bet it was bad.

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