1989 MPFI V6
Engine runs great otherwise. Really well for having 196000 miles. No major problems in the 2 years I've owned it.

One nagging issue. When the car is warmed up and in closed loop
(please bear with me if I have them backwards, I mean the mode where it is listening to sensor readings, in other words at normal operating temperature, and verified via jumping the ALDL and watching the check engine light where it is blinking the SES once per second or slower)
closed loop, and sitting there idling, every 25 seconds, to the millisecond, the idle will stumble for about 1/2 second, then come back strong. SES in "field service mode" indicates that it switches to open loop. (blinking very rapidly) during that 1/2 second it stumbles, then back to closed loop.

If I drive the car, it does not stumble with the gas pressed, only while sitting at a light, and only at a long light. It will not do it immediately at a light, but after I sit at idle for 2 minutes or so it will start.

Its very odd, the EXACTLY 25 seconds part I mean. You can set a clock to it, and about 1% of the time it stalls the car.

Any ideas?

No consistent or persistent engine codes, other than ONCE after the stumble it gave me "oxygen sensor lean" I've since cleared it and over 3 months of heavy driving there has been no codes at all.

 

That's a strange problem alright. Well, let's see....closed loop operation is determined by a couple things. The engine has to be at a certain operating temperature, and the O2 sensor has to be putting out a variable reading, among others things. Now we have no reason to suspect the coolant temp sensor, BUT the O2 sensor did throw a code. Is it the original? If the O2 sensor is old and glitching, it will make the car run rich because the O2 reads leaner than it really is. Hence, the lean codes. So what I'm thinking is that, every 25 seconds when this happens, the O2 briefly stops sending a variable signal, in other words the O2 is 'too cold' to run closed loop, so the computer kicks into open loop.

So in other words....in my opinion, replace your oxygen sensor. At $20 for a new one, if it hasn't been done yet, even if it doesnt solve the problem it's most likely worth it for the gas mileage it'll give you back.

 

I replaced the O2 sensor (Bosch), with the following results.
Idle seems a bit rougher, and now EVERY time the engine stumbles it throws a 44. The engine now stalls about 15% of the time when this happens.
About to do plugs, cap, rotor, coil, wires, fuel filter, PCV valve and do the brakes this weekend, I'll let everyone know.

Its still every 25 seconds at a long light, and it still switches to open loop mode for the 1/2 second it stumbles.

Other posts lead to IAC and TB cleaning, and/or a fuel pump on its way out. (uggh, cuz fuel pump is one of the few things I actually fear doing...)

I would assume that the new sensor is reading the engine condition quicker and more accurately, which is why it throws the code every time now. Now on to diagnose the lean condition.

Anyone have any thoughts on this?

 

Well....the O2 isn't known for throwing ghost codes unless it's old and dying....so I would assume a lean code means you're running lean. Check your fuel pressure. If possible, keep a gauge on it in the driveway, etc. and when it starts stumbling, watch what the pressure does before and after. You're aiming for maybe 35-45 psi of fuel pressure. You're looking for sudden drops or dips.. I doubt a single cylinder going lean would affect the exhaust enough to throw full out lean code....so I wouldnt suspect a problem with any one fuel injector, but rather with the fuel system. You're gonna definitely have to update me after the fuel filter though. That's the logical next step here.

 

My friends car (97 Firebird 3.8) needed plugs today and asked my help as she couldn't get it done (at least not without spending $200!!!!) so I decided to make a day of it and get most of my stuff done today (Conrad dist cap (brass terminals), rotor, Bosch Platinum plugs, fuel filter, clean intake and IAC, check TPS, as well. (Man talk about a pain changing the plugs on that 3800 Series 2, but thats another thread, actually another website )

Well come to find out, my number 1 (not 100% on numbering) passenger side front of engine plug was completely black, and caked with carbon. It was the only cylinder that looked that way. 2 years ago when I got the car we did the plugs (Bosch Platinum as well), but I'm not sure if we did the wires or not. Either way the wire had melted and fused to the exhaust manifold! When I went to pull it out it hung up and then fell apart in three pieces! My guess is, since the car has run the same way since I got it, the wire had been arcing to the exhaust and misfiring randomly since the day I got it. It got dark since it took 4 hours to do the 97's plugs, and I only finished my cars plugs and wires up by the time it got too dark to work.

Man talk about a world of difference with a new (crappy Discount Auto parts OEM set) set of wires and plugs!

Its like I'm driving a whole new car! I always wondered what the hell was wrong with my engine, I seemed to rev very slow when I stomped it, and I always had a good deal of carbon at the tailpipe. So come to find out, I'd been running on like 5.5 cylinders for 2 years now!!! I drove it for about 25 miles after that, with light evening traffic, and holy cow! I think I can keep up with the minivans now! I always figured since the car idled OK and only stalled once in a blue moon I was basically working OK. Unbelievable that I could have been so stupid as to not check the most basic things for funky problems that I just figured was because "thats how my car runs"

Tomorrow comes a lot of driving in rush hour, I'll report if the every 25 seconds problem rears its ugly head again. (I'm predicting it won't because I did have a couple longish lights where normally it would have occured, but I can't be 100% yet.)

Crosses fingers..... (And oh yeah, fuel filter will get done sometime next week as time permits unless the problem happens to me again)

 

the "25 second" problem showed itself today in traffic. Same symptoms as before, only now my car throws a 34, instead of the earlier code. Stalling is more apparent as well (I stalled a total of 4 times in about 100 miles of driving, all on the second or third iteration of the 25 second problem) I checked and rechecked vacuum lines (I had this before and discovered two tiny cracks in the lines from the TB) and there are no apparent vacuum leaks. 34 is MAF on my car, I'll lurk around and see if anyone has anything on the subject. Thanks everybody in advance for any suggestions or tips

 

Ignition module (under the cap) might be heating up on you...change it with the other ignition parts....that many miles hard to say how old it is. They can test them, but have it tested a few times so it warms up good...

 

That 25 second "to the millisecond" interval seems too consistent to be a sensor problem or random glitch. That sounds like it may be a timed function of the ECM for mode check, spark retard attack, or something similar. Do you know what ECM and PROM code you have?

 

Car's CPU is completely stock, can you tell me how to find that?

 

Bob Dole should take the pen out of his right hand, and place a phillips screwdriver, a ¼" nut driver, and 7mm nut driver there instead. Bob Dole should then dive into the footwell of the right side of the car and remove the lower dash trim with Bob Dole's good (left) hand. Bob Dole will then see the lower end of the ECM and mounting screws. Bob Dole should remove the mounting screws and lower the ECM out of the dash. Bob Dole can then read the label on the ECM for part number, and remove the rectangular cover from the back of the ECM and read the foil label on the PROM for revision number. Bob Dole can then return for his pen and write/record the numbers he finds, then report back with that information.

No one is going to get an upper hand on Bob Dole on this one, dad-gummit! Bob Dole will lick this problem yet.

 

AHHH, finnaly any one in the same problem.

Well, I replaced ignition module in my V6 and nothing. Engine starts when cold, run normally for 8 minutes, BLM GO FROM 144 TO 101, and engine dies when hot: No more spark at plugs, no more electronic pulse to injectors.

Then, when engine is cold, starts again normally....., etc (the cycle starts again).

ECM is the 1227730. The problem is the same with PROM or without PROM (in limp home mode), with O2 connected or disconnected.

(One spark plug is black, the others are good). Coils, spark plugs, wires, coolant sensor = only 1 year old / O2 sensor, EGR, ignition module = news.

What could be?: ECM?, cal-pack?, a headache?

This weekend I'll test the ECM from my wife's Corsica (730 too) in my car and I'll see what happens (I hope don't fried this ECM in this test).

Thanks for any replies or ideas.

Denis V.

 

Denis.V:
Um, I am not so sure the ECM swap is a good idea. They might be the same ECM, but I doubt they have the same PROM/mask/whatever. I am not sure though because I am not very knowledgable when it comes to the PROM stuff.

bobdole369:
If you haven't figured out your problem yet:
After reading through this thread, I would not be surprised if it is the same problem I was having with my 2.8L in my RS (also an '89). I was having strange idle problems where the idle would drop way too low, engine would stall or stumble, and sometimes I would get a code 34. Turns out the idle speed screw needed to be adjusted. The procedure is in the Chilton's manual (not in the Hayne's) for 82-92 Camaros and is pretty easy. I wish I could remember it in it's entirety, but I can't. Before doing the procedure, you are going to want to make sure your timing is correct though, as this will effect the idle stability. Also, after you have adjusted the idle screw, you are going to have to adjust the Throttle Position Sensor (TPS) since minimum-air (closed-throttle) position will have changed.
There is a procedure here: https://www.thirdgen.org/newdesign/tech/tpimod2.shtml
for doing this as well, although it is missing some of the idle adjustment steps specified by the Chilton's manual (steps for adjusting TPS are the same). I believe the Chilton's manual also specified different minimum idle speeds (i.e. - not 450RPM).

Let me know if this is still a problem and I will try to remember to get the procedure out of the manual for ya.

 

Appreciate it, as for identifying my PROM, I haven't had a chance since my car needs to be running all the time I've been getting nothing but code 34 in the meantime, I check codes everyday hoping I get something different that helps me, but no luck so far . Typically the SES light comes on not when the idle stumbles, but randomly at any time, usually 2-3 times per day driving 100-200 miles/daily.

Searching through other articles, I found a check for a MAF problem vs. a relay problem, and applied it. Simply swap the MAF and the burnoff relay. If the relay is the problem it should theoretically throw a 36. Well I've been driving for 3 weeks with em swapped, and still a 34.

 

It is throwing code 34 at random...even at cruising speed or under acceleration, not just idle?

 

Have you tried the tap test on the MAF?

 

it throws code 34 at random, either cruising or idle. Tap test on MAF? (gives confused look). I'll try to look it up here, and report back tomorrow.

 

Start and warm the engine. With the engine running, lightly tap on the MAF housing to see if you can generate a stumble that coincides with your tapping. That can indicate a cracked sensing hot wire in the MAF or poor connection to the MAF.

 

A quick techincal note about the ThirdGen V6 MAF sensor:
The MAF sensor used on ThirdGen V6 cars is a frequency-film type MAF, not the hot-wire type. There is a thin peice of film in the MAF that vibrates at different frequencies according to the air flowing across it (from what I understand).

 

OK, here is that idle adjustment procedure, sorry it took so long. Don't know if it'll help in this case, but it worked great on both of my 2.8L Camaros to get the idle back to normal. (Especially the 89 that was stalling. The throttle plate was not even close to being open enough at idle.)

I have done the adjustment twice on the 2.8L V6 (once for each of my Camaros), and it is so similar to the TPI adjustment that I can pretty much say the TPI adjustment will work just as well. As for the 3.1L V6 adjustment, I've never had a 3.1L to adjust the idle on.

My own comments added in italics.

Before you do any of these adjustments, make sure your timing is where you want it. I have found that changing timing will subtely affect the idle.

Tuned Port Injection (TPI)
1.) Using an awl or equivalent, pierce the idle stop plug and remove it. This is the plug covering the idle speed adjustment screw (aka idle stop screw, aka minimum air adjustment screw) on the back side of the throttle body.
2.) Leave the Idle Air Control motor connected and ground the ALDL diagnostic terminal (connect a wire between the A & B terminals on the ALDL connector). Turn the ignition to the ON position, but do not start the engine.
3.) Wait 30 seconds, and with the ignition switch still in the ON position, disconnect the Idle Air Control connector. (Since the A & B terminals are connected on the ALDL, the ECM will place the Idle Air Control Valve in the park position. Disconnecting the IAC connector will keep the IAC at this setting.)
4.) Turn the ignition switch OFF. Disconnect the distributor set-timing connector. (i.e. - The wire on the firewall you disconnect before adjusting timing.)
5.) Remove the ground wire from the ALDL connector. Starting the engine with this jumper wire in place may damage the ECM. See more info from "Vader" here: https://www.thirdgen.org/techbb2/sho...hreadid=211733
6.) Start the engine, then connect the A & B terminals on the ALDL again. This will place the ECM in Field Service Diagnostics mode which will flash the SES light according to open/closed loop status. Allow the engine to reach operating temperature and go into closed-loop operation. Watch the Service Engine Soon light on the dash. When the light is flashing 2-3 times per second (i.e. - rapidly), the engine is in open-loop mode. When the light is flashing about once every second (i.e. - slowly), the engine is in closed-loop mode. Once the engine is in closed-loop mode, continue to step 7.
7.) Adjust the idle stop screw to 400RPM on the 5.0L engine or 450RPM on the 5.7L engine. Adjust the RPM with the transmission in neutral.
8.) Turn the ignition OFF and reconnect the Idle Air Control connector and and the distributor set-timing connector. Remove the A & B jumper wire from the ALDL connector.
9.) Adjust the throttle position sensor to 0.54v at closed throttle since adjusting the idle screw has changed the position for closed throttle. Start the engine and check for proper idle operation. You will have to disconnect the battery to clear the code set from disconnecting the distributor set timing connector.

Multi-Port Injection (MPFI)
2.8L Engine
1.) Using an awl or equivalent, pierce the idle stop plug and remove it. This is the plug covering the idle speed adjustment screw (aka idle stop screw, aka minimum air adjustment screw) on the back side of the throttle body.
2.) Leave the Idle Air Control motor connected and ground the ALDL diagnostic terminal (connect a wire between the A & B terminals on the ALDL connector). Turn the ignition to the ON position, but do not start the engine.
3.) Wait 30 seconds, and with the ignition switch still in the ON position, disconnect the Idle Air Control connector. (Since the A & B terminals are connected on the ALDL, the ECM will place the Idle Air Control Valve in the park position. Disconnecting the IAC connector will keep the IAC at this setting.)
4.) Turn the ignition switch OFF. Disconnect the distributor set-timing connector. (i.e. - The wire on the firewall you disconnect before adjusting timing.)
5.) Remove the ground wire from the ALDL connector. Starting the engine with this jumper wire in place may damage the ECM. See more info from "Vader" here: https://www.thirdgen.org/techbb2/sho...hreadid=211733
6.) Start the engine, then connect the A & B terminals on the ALDL again. This will place the ECM in Field Service Diagnostics mode which will flash the SES light according to open/closed loop status. Allow the engine to reach operating temperature and go into closed-loop operation. Watch the Service Engine Soon light on the dash. When the light is flashing 2-3 times per second (i.e. - rapidly), the engine is in open-loop mode. When the light is flashing about once every second (i.e. - slowly), the engine is in closed-loop mode. Once the engine is in closed-loop mode, continue to step 7.
7.) Adjust the idle stop screw to 500RPM in D if equipped with an automatic transmission or 600RPM in neutral if equipped with a manual transmission.
8.) Turn the ignition OFF and reconnect the Idle Air Control connector and and the distributor set-timing connector. Remove the A & B jumper wire from the ALDL connector.
9.) Adjust the throttle position sensor to 0.54v at closed throttle since adjusting the idle screw has changed the position for closed throttle. Start the engine and check for proper idle operation. You will have to disconnect the battery to clear the code set from disconnecting the distributor set timing connector.

3.1L Engine
1.) Disconnect the negative battery cable. Using an awl or equivalent, pierce the idle stop plug and remove it. This is the plug covering the idle speed adjustment screw (aka idle stop screw, aka minimum air adjustment screw) on the back side of the throttle body.
2.) Ensure the throttle or cruise control cables are not holding the throttle lever from returning fully. Back the throttle stop screw out until and air gap is visible between the screw and the throttle lever.
3.) Turn the screw in until is just contacts the throttle lever; then turn the screw in an additional 1 1/2 turns.
4.) Connect the negative battery cable and connect a suitable scanner to the ALDL connector to monitor the Idle Air Control (IAC) valve counts.
5.) Place the transmission in P if equipped with an automatic transmission or neutral if equipped with a manual transmission.
6.) Start the engine and allow it to reach normal operating temperature and enter closed-loop operation.
7.) Monitor the IAC valve counts with all accessories OFF, the IAC reading should be 10-20 counts. If not as specified, repeat the procedure.


Edit - 9/6/2004:
Changed idle RPM numbers for 2.8L engines to 500 and 600 RPM from 550 and 650 RPM (these are the numbers found in the GM service manuals).
Added step instructing removal of jumper between ALDL A & B terminals before starting engine (due to possible ECM damage). Added step instructing the connecting of A & B terminals after engine is running to place the ECM in Field Service Diag. mode.

 

WOOO, excellent!.

My only question is for step 7) TPI & MPFI:

RPMs must be 400/450/550/650 a normal temperature range, but with electric fan ON or OFF (running it or not)?.


Thanks,

 

Once at step 7 (for TPI and 2.8L), I would adjust the idle screw only when the electric fan is off. If it comes on, I would wait until it goes off and then continue adjusting.

 

Odd. Nothing has changed anything about this issue until I had the MAF disconnected, and I replaced my ignition coil. With that done, it really doesn't ever do it. (scratches head).
Plug in the MAF and it does so ever so slightly. Need to replace MAF anyways, so I think it'll go away with a new one.

 

You guys miss one key thing. The coolant temp sensor(s) and part of that circuit as well, and that's what tells the o2 sensors to go into open or closed loop. Alot of the systems on the car work together. When the engine/coolant is cold, the motor needs more fuel (open loop status), When the engine/coolant is up to operating temp it needs less fuel(closed loop status)
Is your car always in closed loop status? If it is then it's telling the 02 sensor the car is always running lean.

 

I had some idle issues with my Camaro's intake air temperature sensor as well. It was reading 75F when it was 30F outside, so the car ran horibbly during the first 10 minutes after a cold-start.

Closed loop status does not mean that you are running lean. Closed loop means the computer is using the O2 sensor pretty much exclusively to determine fuel mixture (and whether to richen/lean it out). In open loop, the computer is using reads from the TPS, MAF, coolant, etc. to determine fuel mixture.

Closed loop gives better fuel economy because the computer can actually "see" how the fuel is being burnt/used and if it is feeding in to much/little. In open loop, there is no feedback to the computer concerning whether or not the determined mixture is best.

The engine should only be running in open loop after start-up. The computer always enters closed loop after some conditions are met. I can't remember what all they are (I think there are 3). One of those is coolant temperature has to be at a certain level. Also, the engine has to have been running for a certain amount of time...I can't remember the third. Blast it. Anyhow, once all three of these conditions are met, the computer should enter closed loop mode.

 

Aye, tis correct. I've shorted the pins and drove around with it like that to observe closed/open loop status.

It never happens in Open loop.

While in closed loop if I happen to idle for a while, I can see it enter open loop for about 3/4 second (very fast flashing). (enough to get to the "OU" in one-one thousand") EXACTLY when the idle stumbles. It recovers immediately, and the check engine light also follows, and indicates closed loop operation. This repeats to the MILLISECOND every 25 seconds. I've sat in my driveway with a stopwatch and timed it.

With the MAF disconnected it simply doesn't happen. (I'm guessing because it is always in Open loop mode) However the power loss, and fuel inefficiency are enough to make it too difficult to live with it.

Any easy way to check coolant temp sensor operation?