Rough Idle, HELP! - Team Camaro Tech
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post #1 of 16 (permalink) Old May 11th, 00, 05:01 PM Thread Starter
 
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I have a 1991 Camaro RS w/TBI 305ci.
It starts but surges while idling. If I give it gas, it can't sustain the RPM, it will slowly die out. It has a good fuel filter/pump/regulator, I have thoroughly cleaned the TBI unit, I checked the vacuum relay switch, and replaced the EGR valve, and none of the above have changed anything. It does this with the stock chip and the hypertech. If I unplug the vacuum relay there is no big difference. When I give it gas, the injectors seem to be firing off plenty of fuel. I have checked vacuum at several points around the TBI and it is in normal ranges, but I still suspect a possible leak. What could this be? The problem didn't happen gradually, it ran fine one day and then it all went to hell at the same time. Any help would be greatly appreciated!
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post #2 of 16 (permalink) Old May 12th, 00, 04:47 AM
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Other than a vacuum leak have you checked your throttle position switch? Did it get wet (steam cleaned the engine)? When they go bad they provide the symptoms you're describing.
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post #3 of 16 (permalink) Old May 12th, 00, 06:59 AM Thread Starter
 
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I have never cleaned the engine other than spraying carb cleaner around that area but maybe some got into it, is the throttle position switch that big thing (1 1/4 inch roughly)that screws into the left side of the TBI with a large square 4-pin plug going into it?
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post #4 of 16 (permalink) Old May 14th, 00, 08:44 AM Thread Starter
 
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Throttle Pos. switch is good. I also noticed that the rough idle starts only after warmup. Once the electric choke disengages after about 1 minute, this is where it cannot rev up. Also, when the vacuum relay switch is unplugged, it becomes driveable. It will hold the rpm and not bog down. Like this it has absolutely zero power, but it doesn't die when I give it gas. This is really wierd!
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post #5 of 16 (permalink) Old May 14th, 00, 12:39 PM
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You might check you oxygen sensor also. The car doesn't use the O2 sensor until it's reached a certain temperature; before that it uses the map/maf, tps, iat, etc. Once it reaches a pre-determined temp. it goes into closed-loop where it uses the O2 sensor to maintain mixture. When an O2 sensor is bad it doesn't affect cold operation as the computer isn't using the O2 sensor until warm.

By the way, the best bet is probably having a shop check it out. It get's expensive replacing things that aren't bad. However, the O2 sensor isn't much money and that would be my next choice.

P.S. Unplugging the relay PROBABLY puts the computer in "limp-home" mode; this mode doesn't usually use the O2 sensor, just goes with basic tuning input, less spark advance, non-optimum fuel tables, etc. just to get you home. This mode tries to protect your engine; with the relay un-plugged the computer isn't getting all the info it wants so it tries to protect itself. It does reduce power in this mode, sometimes a lot.

[This message has been edited by Camcojb (edited 05-14-2000).]
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post #6 of 16 (permalink) Old May 15th, 00, 06:37 AM Thread Starter
 
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Yes you are definitely right. It only has trouble in closed loop mode so I suspect it must be one of the many sensors. I'll just take it to have the error code checked by an diagnostic machine. Thanks, you've been a great help!
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post #7 of 16 (permalink) Old May 15th, 00, 03:21 PM
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If you first start the engine and then jumper the two terminals on the top right hand side of the ALDL connector under the dash, the check engine light becomes an indicator of the o2 sensor. The light will stay on while warming up. When it gets to closed loop, it will cycle off and on. One way is rich, the other is lean, I can't remember which, it's normal for it to cycle slowly on and off. You can drive it while watching what it does.

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post #8 of 16 (permalink) Old May 16th, 00, 06:45 AM Thread Starter
 
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I changed the O2 sensor yesterday and it didn't help. I am just about to take it to get it diagnosed. I hope I make it, Limp mode doesn't let me get above 2000rpm or above 35-40 mph!!!
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post #9 of 16 (permalink) Old May 16th, 00, 08:35 AM
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Steve,

If you get a chance, let me know what it ended up being. Really curious now!
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post #10 of 16 (permalink) Old May 16th, 00, 12:25 PM Thread Starter
 
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Hey whats up, just hooked up a scanner and got trouble code 33, MAP sensor voltage too high. The car does not idle anymore when it is hot, so I could only get a voltage reading at about 1100-1200rpm (a bit high for idle) and it showed about 2.5 volts. Normal idle voltage from MAP should be between 0.5-1.5 volts, so it is way high. When the throttle is touched, the voltage jumps up to about 4V and when throttle is let go it sticks at 4V for a couple of seconds and bogs the engine. The MAP sensor doesn't leak, it holds the 15psi vacuum I tested it with. The vacuum line entering the MAP seems to be steady, but fluctuates periodically. So my next step is to verify that the vacuum line entering the MAP has no leaks, and if not then it is safe to assume that the MAP is the problem. The ONLY thing that makes me doubtfull is the performance of the car right now. Before, with the vacuum relay solenoid plug pulled out to turn on the limp mode, the car would not bog as easily and would do a max of 2000rpm with zero power and reach about 35mph. When I was driving home (about 1 mile) the car would only give me 1200rpm, any more throttle and it would die! I could only drive at a walking speed! And if the engine got past 200 degrees, it would die and not start untill temp. dropped back down at least 50 degrees. I had to stop and let it cool twice so it took me an hour to get home!! Can a MAP sensor malfunction do this??!! I'll let you guys know what happens.
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post #11 of 16 (permalink) Old May 16th, 00, 01:08 PM
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It sounds like something beyond just the map sensor. I had a 383 93 Vette and I played with the map sensor a lot; even rigging up things to trick the sensor into thinking there was more vacuum than there was (adjustable MAP sensor). Point being I ran the car without it even hooked up (by accident). It ran a lot better than what you're describing. Limp-home mode is just a set of fixed inputs to get you home; you do lose some performance, but not to the point your's is doing.
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post #12 of 16 (permalink) Old May 16th, 00, 06:14 PM Thread Starter
 
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Surprise surprise, with my luck how could I have ever thought this would be it! All vacuum lines are intact and reading about 18psi, timing is on the money and it is NOT the MAP sensor =( I would suspect internal problems but when it's cold, the car starts and idles smoothly. It MUST be electrical. Well i've changed the EGR, O2, MAP, vacuum relay solenoid, fuel filter, and i've checked all vacuum lines/readings, throttle pos. sensor, fuel pressure regulator, rebuilt the TBI, and the only trouble code that showed up was the MAP (before replacing). Well I'm flabbergasted! I will most likely be towing it to a dealership later this week to get a full diagnostic. I'll post what the problem was when I find out, but it'll probably be several days since I am busy with school finals/work.
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post #13 of 16 (permalink) Old May 17th, 00, 04:52 AM
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LT1Steve, after all of your checking, I think your problem is the cat converter, if the converter is connected with a flange, disconnect it and see how the engine runs. Keep us posted.

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post #14 of 16 (permalink) Old May 18th, 00, 04:28 PM
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Chev64,
I think you may be right. It would have very low vaccum when trying to rev it too.

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post #15 of 16 (permalink) Old May 18th, 00, 06:14 PM Thread Starter
 
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I'll give it a try before I tow it in, nothing to lose. I really don't think so though, because the bad running definitely starts right when the choke turns off after warmup. With a plugged cat I would expect it to be bad the whole time it is running. But I do know that recently about 1-2 weeks before the problem started, I was pulling into a wierdly shaped driveway and the cat hit harder than ever before, so hard that it put a small dent into it and backed it out of the y-pipe a couple of inches. So if there would ever be any problem with it, it would be now. I'll pull it off and crank it tomorrow. I have headers so itsa gonna be loud!
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