Was hoping some could provide input/insight into a problem I'm having. I have a 1987 fiero with a 1987/1988 Buick 3800 V6 in it. Car has been running fine, and now since yesterday, it will have intermittent periods of spitting/sputtering and occasional backfire, and then it will clear up and run like nothing happened at all.
The problem has gotten worse, and I have backprobed the TPS with some strange results. The key on engine off gives about 5V reference voltage (ie input), and then when I start the car and let it idle, the reference voltage (input from computer) will slowly increase as it runs. Seems like 6V input to the TPS is about the point where the spitting/sputtering starts and the engine stalls. When the engine stalls the input voltage drops back to 5V, and if I restart the car it will do the same thing again.
Any ideas on if this is a PCM issue, TPS issue or something else electrical related?
On a side note - I have checked fuel pressure - gets 40PSI key on engine off, and 32PSI at idle. When it starts to stumble the PSI jumps up a couple to about 34PSI. If the engine recovers, the PSI goes back to 32, and jumps up again when it starts stumbling. Also, the fuel filter is brand new (put it in about 100 miles ago, so I'm inclined to believe its not fuel related because of those facts and that you can smell the car is running richer the longer it idles (which makes sense as TPS output is proportional to input, so the computer receives input that leads it to think the throttle is being depressed.
This doesn't appear to be vacuum related as it runs/idles great while the TPS voltage is in normal range (ie near the 5V), and also if I remove the input hose to the MAF and place my hand over the MAF input, the car stalls out almost immediately.
Any ideas?
[This message has been edited by nyjetfan (edited 11-06-2013).]
Well just did some diagnostics, and backprobed the TPS. Was reading 0.25V, so I adjusted it up to 0.52 (should ready about 0.5 at idle setting). Started car and it started to do it again. Probed the TPS and it was reading almost 1.5 Volts! I probed the power source for the TPS and it was reading 6.12 Volts, but I thought it *should* be a 5 Volt reference point? As I watched the backprobing of the TPS, I could see the signal voltage bouncing and the reference voltage (input from ECM) would bounce mostly between 5 and 7 volts (it should be a 5V reference based on my knowledge) and occasionally when the car was sputtering it would drop down to like 1-2 volts.
Going to try checking with key on engine off again when it cools down, but if I'm really seeing 6.1 Volts at the reference for the TPS, would that mean I have a bad PCM/ECM computer module? Any way to test this suspicion?
Update: It seems to read about 5V when the key on engine off, but when the engine is running the PCM reference voltage jumps all over the place (as mentioned above). Does this warrant a new PCM/ECM module? The one in there now is an AC Delco ECM Reman - # 88999147 - any way to test this unit to confirm its faulty other than swapping?
[This message has been edited by nyjetfan (edited 11-05-2013).]
What service number is on your ECM you are using with your 87-88 3800 engine?
Concerning the 5v reference from the ECM - it should always be a rock solid 5 volts and it is supplied by the ECM via internal voltage regulator. If you see 6 volts on any 5v reference wire, then either you have a short in that wire to another power source or the ECM is bad.
-ryan
------------------ OVERKILL IS UNDERRATED Custom GM OBD1 & OBD2 Tuning | Engine Conversions & more | www.gmtuners.com
[This message has been edited by Darth Fiero (edited 11-06-2013).]
Where would I find the service number? The ECM is an AC Delco and it says the following on it On left side of decal reading down) ACD# 88999147, QM# 88999147, Qty 1, ECM-REMAN,
On right side I see this: LC21 4907 DCA4344 GR3.670.CG.ECTH REMAN IN USA
Is there something else I need to look for to get the service number? At this point I'm debating whether I should just drop the $75 and pick up a new ECM, or if I should try to run a new feed wire to the TPS first. My gut is that its the ECM, as further testing I see it start at 5V key on, engine off, and then after starting it I can watch the voltage slowly climb over time. If it was a short, I'd expect it to jump to the higher voltage immediately upon starting.
Also, just for giggles, I disconnected the alternator (ie ran on just battery) and it ran fine, although the TPS input was a little low - in the neighborhood of 4.85V, but it consistently stayed within +/-0.02V of 4.85V while I ran it. With alternator plugged in, I can see it go up a maybe 0.1V every 5-10 seconds, and once the input hits about 6.0V, the spitting and sputtering starts and the exhaust smoke starts turning black (No black smoke to start, and as the input voltage goes up, the smoke starts turning black). Battery voltage with key off is in neighborhood of 12.7-12.8V, and when running with alternator on the battery reads 14.0-14.1 Volts, so both of those seem to be within expected parameters.
[This message has been edited by nyjetfan (edited 11-06-2013).]
Where would I find the service number? The ECM is an AC Delco and it says the following on it On left side of decal reading down) ACD# 88999147, QM# 88999147, Qty 1, ECM-REMAN,
I think p/n 88999147 crosses to an OEM Serv.No. 1227783 which was used in the 87-88 3.8L LG3 applications. This ECM should have a removable EPROM chip that looks similar to what the stock Fiero ECM uses. It should also only have 2 black connectors and basically look almost identical to a stock Fiero 2.8 ECM.
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Is there something else I need to look for to get the service number? At this point I'm debating whether I should just drop the $75 and pick up a new ECM, or if I should try to run a new feed wire to the TPS first. My gut is that its the ECM, as further testing I see it start at 5V key on, engine off, and then after starting it I can watch the voltage slowly climb over time. If it was a short, I'd expect it to jump to the higher voltage immediately upon starting.
Also, just for giggles, I disconnected the alternator (ie ran on just battery) and it ran fine, although the TPS input was a little low - in the neighborhood of 4.85V, but it consistently stayed within +/-0.02V of 4.85V while I ran it. With alternator plugged in, I can see it go up a maybe 0.1V every 5-10 seconds, and once the input hits about 6.0V, the spitting and sputtering starts and the exhaust smoke starts turning black (No black smoke to start, and as the input voltage goes up, the smoke starts turning black). Battery voltage with key off is in neighborhood of 12.7-12.8V, and when running with alternator on the battery reads 14.0-14.1 Volts, so both of those seem to be within expected parameters.
If you measure voltage between the black and grey wires going to the TPS, you should get very close to if not dead spot right on 5 volts at all times the key is on and if the engine is running or not.
Something else you might want to check is ground. Measure voltage from the TPS black wire to engine block ground. It should be very close to 0.0 volts as displayed on the volt meter at all times (key on, engine on or off). If you see voltage when doing this test, you may have a ground problem (check ECM to engine block wiring harness grounds).
While you are at it, check the ground between the block and the chassis and between the block and the battery negative terminal. Voltage readings across these points should be very close to 0.0 volts. I think the acceptable maximum level is 0.3 volts difference. If you see anything higher with the engine running & alternator charging normally, then you probably have a bad ground connection and you will need to fix that first.
If all your grounds check out and you are still not getting 5 volts on the grey TPS wire, check it at the ECM (backprobe connectors) and verify you are still having trouble there. If so, I suspect your ECM is bad. I do have spare used 1227783 ECMs on hand and could ship one to your door for less than $75.00.
Thanks Ryan - Sent you a PM about a possible spare ECM, although I think you nailed it here:
"Something else you might want to check is ground. Measure voltage from the TPS black wire to engine block ground. It should be very close to 0.0 volts as displayed on the volt meter at all times (key on, engine on or off). If you see voltage when doing this test, you may have a ground problem (check ECM to engine block wiring harness grounds)."
I backprobed the TPS ground, and with key on engine off, I was getting a reading bouncing between -0.01V and 0.01V on my multimeter, which I thought was close enough to zero. However, when I started the car, this jumped up to 0.62V, which is NOT close to zero in my opinion. Do all 3 of the TPS wires feed back to the ECM (I know the reference and signal voltage have to, but not sure about the ground).
Edit: Just did reference and ground and they are different by almost 5V on the nose as long as the key is on (engine running or engine off). Looks like the PCM ground is possibly bad/rusted.
Any idea where I can find a diagram for this ECM or would you know what wires are the ECM ground wires? Appears there could be something odd/electrical going on, so I need to find some time to remove/clean all the grounds, but as I mentioned earlier, I'm also interested in a backup ECM to keep in my "toolbox" (along with a couple spare coils and an ICM).
This is where the fun begins, as the electrical problems are never any fun. Any way to check these using an ohm meter (ie how much resistance) as that is a little easier to do in advance of starting the car.
[This message has been edited by nyjetfan (edited 11-06-2013).]
Update #1: Just got a new negative battery cable and put new engine block ground in place. There was a wire tapped off running inside the car which looked pretty nasty, so I cut it off, stripped a new section and put a proper end on it to attach with the ground bolt to the block. The original negative cable was a little loose in at the block side (I could wiggle it back and forth with the nut tight, and whoever worked on it before stripped the head pretty good, so I just pulled it out and replaced that, and cleaned up the end of the other.
Started the engine and the TPS input reference voltage is now 5.25V, and the ground wire reads about 0.25V. I took the car for a drive, and I have to say it was much improved. The throttle response seemed much better, seemed to have a little more power (granted this is still a sotck 1988 Buick V6, so its by no means a rocket).
At this point I'm trying to research the ECM pinouts, so I can find the ground there and see if there is a ground issue between the PCM and the engine block. I do have a nice ground from the engine block that is terminated inside the console, so as the PCM sits behind the drivers seat, if I find the grounds aren't great, I can always pull them over to this ground for a test.
If anyone has the pinouts/wiring diagram for a 1988 Buick LeSabre/Century/almost anything Buick V6 fuel injected from that year, please let me know where I can find them.
(Figure 5 - 1986-1988 Buick LeSabre) Looks like Black connector has A12, D1, D6 and D7 to ground and Orange connector has D3. How are the pins numbered on the ECM plugs? It says all the grounds are Blk/White wires, but as this was an engine swap of unknown origin, I'm hoping to get a little more definitive on what pins should be grounded so I can test them.
[This message has been edited by nyjetfan (edited 11-07-2013).]
Update #2 - Took my diagram out and was able to work my way through the wiring and find a ground. Probed it with key on engine off and also engine running and it was about 0.21V with engine running (same as I was seeing on the TPS). Grounded that wire better and now TPS reference voltage reads 5.11V with key on engine off, and 5.13-5.14 volts while running. I can say the spitting and sputtering is gone and throttle response is improved, although I think that can be attributed to the new fuel filter I put in about 1-2 months ago as well (I changed that when I was having similar problems in the past).
Thanks Darth Fiero (Ryan) as you nailed this issue and enabled me to get my car from a spitting/sputtering clunker to driving smooth again.
Originally posted by nyjetfan: Thanks Darth Fiero (Ryan) as you nailed this issue and enabled me to get my car from a spitting/sputtering clunker to driving smooth again.
