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Discussion Starter · #1 ·
Car was running fine, drove about 50 miles shut it off to run into a store, started it back up and when pressed the throttle it was like it was getting no fuel. Idle was fine but no go when pedal depressed. After several attempts at giving it throttle it picked up and went only to fall on it's face about 1/4 mile later. Same thing will not take throttle but idles fine. I checked several things under the hood and kept trying suddenly it would go but then fall on it's face again. In total I made it about 3 miles before having it towed and had to pull over about 6 times over that distance.
 

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Discussion Starter · #2 ·
Continued .... So once I got it home I hooked up my scanner and started to run tests on it. Once the engine cooled down even getting it to idle was quite a challenge. It would start without issue and idle fine for a couple of seconds then idle way down 300-500 rpms, very rough and then stall. Would not take any throttle. After about 5 or 6 starts it would try to keep running and eventually I got it to warm up and then it would idle again but not take throttle most of the time. If I unplug the MAF then the car will take throttle as it should but with it hooked up it takes it maybe 1 out of 50 times. Sensors are showing very high STFT +40% both banks with MAF connected.Tested fuel pressure with a gauge reading all look correct 32-40 PSI depending on vacum. Air flow rate is odd showing 0.4 lbs/min often at idle where before this problem it never dropped below 0.7 and was 1.0 or higher if the engine was over 1000 rpm but now even at 1500 rpms sometimes it reads 0.4 other times it may read as high as 1.1 at the same rpm but in both cases the STFT is very high and car refuses to take throttle. I replaced the MAF, no improvement. If I unplug the MAF the idle is a bit erratic but the car responds well to the throttle. STFT drops to negative numbers. OS2 sensors seem to be working properly assuming of course I am interpreting the data correctly. I also ran some voltage tests on the MAF leads and get conflicting info. At one point I was only getting 3.8 volts for battery + to signal ground then I was getting 12.xx volts. connectors look clean and shiny, no visible issues there but that voltage reading would seem to be a problem. I am starting to think maybe the ECU is the problem but am reluctant to just buy one and cross my fingers.

Any one have any idea what may be going on here?
 

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Have you checked for breaks in the wiring to the maf? It sounds to me like the maf or associated wiring. Try the test you ran again but this time wiggle the harness with the probes in and watch for voltage fluctuation.


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Discussion Starter · #4 ·
Thanks I'll dig a little deeper on the wiring side. Maybe pull the ecu and check the resistance on the wires going to the MAF and grounds to the ECU.

Good point about wiggling them a bit it is possible that may show something..
 

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Discussion Starter · #5 ·
Initial tests show both the MAF signal and signal ground both have good connectivity back to the ecu <1 ohm on each
 

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How about the signal return?


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Discussion Starter · #7 ·
Both wires tested good back to the ecu. After testing that I reinstalled the ECU and put back the original MAF. Tested the signal ground and got 12v with engine not running and 14v when running. wiggled the wries a bit no change. Attached across the two signal wires and started the car. Got about 1.1v at idle and voltage raised as I tried to give it throttle so it seems that both the MAF and the wiring are ok. I also did some more testing of the o2 sensors. Forcing a lean condition by pulling a vacuum then rich by spraying some carb cleaner into the open vacuum line both case the o2 sensors responded. I took the ecu apart and inspected it, nothing visibly wrong. no burns, moisture or anything obvious there. In the diagnostic section of the app I am using it showed 4 tests failed to related to rich lean sensors and two that just showed up as CID $11 and $12 with the test $41 showing a value of 117.xxx. The car runs extremely lean when the MAF is connected and rich when it is not according to the STFT and the O2 sensor output. Seems like I have tested everything that I can and not found an issue other than what I am seeing in the data stream from the ECU. The fact that it seems to rev just fine with the MAF disconnected and will not when it is connected along with the fact that the new MAF did not help and even though the voltage readings from the MAF seem correct as the throttle opens the data from the ECU does not. I am leaning more toward the notion that it must be a fault in the ECU. Can't think of anything else to test at this point.
 

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I've been watching close as I have a '95. No fuelish smells huh? I know, your '96 is different.

I had a problem a number of years ago with a rubber vacuum tee behind the TB ... it was burnt through. Dorman 47072 fixed it. I also had a bad MAF once, but I forget all the symptoms. Wish I could help.
 

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Discussion Starter · #9 ·
I've been watching close as I have a '95. No fuelish smells huh? I know, your '96 is different.

I had a problem a number of years ago with a rubber vacuum tee behind the TB ... it was burnt through. Dorman 47072 fixed it. I also had a bad MAF once, but I forget all the symptoms. Wish I could help.
I've had lots of vacuum issues along the way. first the one behind the throttle body, then the one to the evap then the pcv. I finally just replaced all of the rubber vacuum lines. Even the ones that had not failed were in very poor condition just touching them would leave quit a bit of black residue on your fingers.

I did have an ECU failure on this car about 5 years ago. It was a major ordeal trying to pin it down, I could not figure it out. Towed it to a garage they tried, had it for about 3 weeks and could not figure it out, finally towed it to a dealer and they had it for two weeks before they told me it was the ECU and of course they wanted $800 to replace it with a remanufactured unit I had it towed home, bought a reman unit from NAPA for $125 and all was well for almost 5 years then suddenly out of the blue it just fell flat.

Given that I have run out of things to test and towing it to a dealer and having them look at it would cost more than an ECU I have just went ahead and ordered an ECU. Roughly $200 with the core so we'll see soon either I'll have a running car and send the core back or I'll know it's not the ECU and will have a spare on hand if I ever need it ;)
 

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Discussion Starter · #10 ·
btw I have noticed some fuel smell a couple of times when trying to cold start it but most of the time I do not.
 

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It does sound like a vacuum leak. Maybe try a smoke test of the manifold itself and the hvac vacuum lines in the car.


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Discussion Starter · #12 ·
I would think a vacuum leak would show up easily in the fuel trims showing rich at idle and leaning out as you open the throttle. I would also expect a vacuum leak to cause the most problems at idle and very little when you rev the engine up. I would also expect that if there was a leak bad enough to render the car undrivable that it would have to be a big one and be easy to hear when under the hood at idle and even then if the car would idle which it probably would not that it would take throttle and at high throttle become almost a non issue.
 

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Discussion Starter · #13 ·
btw I have checked repeatedly for any vacuum leaks. I do not have a smoke tester so that was not an option. But I have tested some of the lines and components with a vacuum pump and have used a torch where possible to see if any change as well as carefully listening in various places of course. You can hear some air moving but it seems the sound is from inside the throttle body then also can hear it going through the IAC when it opens. Keep in mind this TBird is a pretty quiet car.
 

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2 possibilities, fouling catalytic converters or wire chaffing. I'm skeptical of the cats, but it's still a possibility. If the cats cause a restriction (can anyone say "Banana in the tail pipe"?), it'll load the engine, reduce the air flow, and the engine will respond accordingly. When you disconnected the MAF, the engine is a positive displacement pump, the ECU will likely drive it as such.

The second thought, if there's chaffing, enough to cause a slight ground but not enough for a full short to ground, it'll throw the signal off. If you have a decent meter that reads 50000 ohms or better, try disconnecting both ends and the designated ground and test the wire to ground.

Also, check your plugs, see what they are telling you. If they say rich, the cats step up a little on the likely suspects.
 

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Discussion Starter · #15 ·
Do you think a cat could have that large an impact that suddenly? I mean the car seemed to be running perfectly, took it on a 50 mile trip. stopped a few times where I shut the car off then started it back up. No issues at all over 4-5 stops then I came out of the store started the car and it had gone from running great to barely running at all. I am far from an expert on these newer cars (by newer I mean any using an ECU) nor am I an expert on cats but I have had a fouled cat on a couple of other cars in the past but never heard of any thing that behaved quite like this and to have it happen so suddenly seems odd. I did have a work van where it had been lacking on power for a while and then one morning it just wouldn't start which turned out to be the cat, I had a car where the engine would not rev above 3000 rpm even in neutral, turned out to be a fouled cat.

If there was a problem with the cat is there something I should look for in the downstream o2 sensors maybe compared to the upstream that would point in that direction?
A shorted wire on the other hand sounds like a greater possibility as if one were to shift and touch the hot exhaust a problem could occur very suddenly. I'll take a look under the car and see what they look like.
 

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Had a truck do it to me once, so yeah.I'm not sold on it being the cat, either, but it's in the list of possibilities. The media is in some form of cage and if it suddenly collapses/fails, you symptoms are going to be sudden. I hate putting on a blindfold and throwing a dart to see which part I should replace next. Quick test is to smack it with a mallet when it's warm to see if the problem relieves itself. If it doesn't, it still doesn't rule it out as the media may not want to move, but if things do improve, you have your culprit. The other option is to disconnect the cat, but that probably involves cutting bolts and making sure you have the right donut gasket.

As for the chaffing. you disconnected the wires, you know how difficult it is (or isn't) to isolate the wire run to the MAF. Ultimately, it's your call as you've experienced the symptoms, whereas, I've only gotten your report. When I hear what you test, I look for other items that may cause interference. Those three items I'd look at before replacing the ECU. If your plugs show lean, I'd look hard at the wires first, if they show rich, that could be cat, but still could be wire.

BTW, did you ever clean either of the MAF's? I've been told that you should clean a new one before installing.
 

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Discussion Starter · #17 ·
I did clean the original MAF as one of my first steps. It looked fine but I cleaned it anyway. I did not try cleaning the new one. After the new one did not help I did carefully inspect the original MAF determined it looked just fine and put it back on the car, ran the test with the meter and it passed them all without issue yet the air flow rate reported by the ECU is erratic and very often lower than expected. For example I have a log saved from when the car was running good. It shows airflow rates in general are always at just over 1lb/min per 1000 rpms The logs after the issues appear show lower air flow rates most of the time sometimes drastically lower and many times they change for no apparent reason but there is often no noticeable difference in the way it is running.

In the old log where it ran good for if the engine was running at 2000-2100 rpms or so I would see an airflow rate of 2.2 - 2.3 In the log I captured yesterday I managed to coax it up to a little over 2000 rpms and kept it there for about 25 seconds during which time I had air flow reports of 1.5 1.7 2.3 and 1.1 it did stay at 2.3 for about 6 seconds of the 25 second run but should have been above 2 the whole time and and one point I had it running at 2400 rpm but the air flow was only reporting 1.5 lb/min.

My current thinking is this if the car is taking in say 1.0 lb/min the ECU should give it x amount of fuel. For simplicity say say x=100 droplets. So then if the ecu thinks it is only getting 0.5 lb/min then it is only going to give it x/2 or 50 droplets of fuel then the o2 sensor says hey I am running very lean here so the ecu says ok I can give you 42.2% more fuel (22 droplets) which would be (x/2)+(x/2*0.42) or ~72 droplets where you need 100 droplets so you still end up with not enough fuel and when you try to give it throttle it just gags on air wanting more fuel that the ecu is not giving. To me that seems to make sense.
 

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Discussion Starter · #18 ·
Looks like it was the ECU. I got the one I ordered today, installed it, started the car ran, a few tests. Responded to the throttle ok, air flow rate looks good, fuel trims look good. Took it out for a short drive and seems to be running great again.
 

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Discussion Starter · #19 ·
Well all was good for a while but then a couple of days ago I was out in the car, car was running fine then suddenly started gagging on throttle. This time I was out in a rural area and just started up a mountain. Fortunately I found a spot to pull over before I lost all forward momentum. I pulled the connector to the MAF and was able to drive it home. Haven't did anything to it yet but the way it acted seems very much the same as before. I am thinking that there is an issue in the wiring somewhere that caused the problem in the first place and may have killed another ECU. I had inspected the wiring to the MAF before as best as I could and did not find any issues with it. It seems that the ECU is miss reading the air flow and not allowing enough fuel but when the MAF is disconnected then it runs pretty well for the most part.

I did put a new intake on the car a while back, do you think it possible that the root of the issue may be in the wiring to the injectors? I am thinking since they have been disturbed a few times then there could potentially have been a problem with one of them.

Any other ideas as to where to look for an issue that would cause this kind of problem?
 

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If it were the injectors or the the intake manifold, disconnecting the MAF would not likely change the performance. Did you ever do anything with the catalytic converters?
 
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