formula_pilot

’89 Formula (LB9) recently began to buck and occasionally stall when tipping the throttle back in after a period of deceleration. The car is a basically stock 5 speed. I datalogged and found that the reported MAF airflow is reading low right after tip-in, but the MAF, and all other factors appear normal during in all other conditions.

For example, if driving at 50mph, then let off the gas, but leave the car in gear, and do this for 3-5 or more seconds, the MAF will read typically between 5-10gms/sec (depending on IAC postion), injector BPW is zero due to decel enlean (all this is normal). But then, when I tip the throttle back in, the MAF data actually reads LOWER, (4-5gms / sec), and the injectors only fire at a tiny BPW, 0.7-0.8ms, the car bogs horrible, then after a second or two (sometimes longer), the MAF reading will jump to 30-50gm/s , the injectors fire normally, the car jerks ahead, and drives normally. It only does this after a period of several seconds with the foot off the throttle and the engine in decel enlean. No other tip-in issues, OK off a normal idle. TPS voltage appears to follow throttle correctly. I did not get any codes from the ECM, even when TPS read 2.5V and MAF was 4 gm/s for several seconds.

I knocked on the MAF with the engine idling, and there was no effect on idle. The burn-off relay is less than a year old, and swapping MAF and MAF burnoff relay had no change.

Is there any way that the MAF could go bad only under this very limited condition? Could a bad ECM do this?


The only other thing I could think of, is if oil was getting sucked off of the K&N air filter when tip-in occurred, then maybe the wire is getting coated with oil for a few seconds, then clears and the MAF is happy again. The problem with that theory is- it has been over a year and 3000 miles since I last cleaned and oiled the filter. I think if there was going to be a problem due to oil on the K&N, it would have manifested right after servicing the filter, not now.


Anyone ever see a TPI engine behave this way? Right now, I am planning to go ahead and change the MAF anyway, but I do not understand how it could only misbehave under this limited condition.

formula_pilot

NG

I replaced the MAF Sensor with a new Wells SU145 (Bosch) unit. Problem is still there. The cutting-out is getting worse, now the car will sometimes complety stall after tip-in following decel. No codes thrown.

My next thought is maybe a bad ECM?

formula_pilot

Changed the ECM, problem still there.

Is there anything on the Memcal board that could do this? I am running a Moates Ostrich, but swapped to a actual chip, and the problem is still there.

Does anything else besides the chip on the Memcal board affect normal operation, I understand the "limp home mode" is on the Memcal on a 165 ECM?

I am running out of ideas on how to troubleshoot this.

formula_pilot

I ran test leads directly to the MAF connector, and drove the car while watching a Voltmeter. When the car stumbles, the MAF signal (green vs black wires) goes to 0.02 volts. I also checked the 12V power at the MAF, and it is holding a steady 13+ volts during the event, so the MAF is getting steady power, but the output goes to near 0 volts.

MP81

How about the MAF connector?

formula_pilot

Good idea. After running tiny wires with a Voltmeter directly into the MAF connector itself, it read it pretty much rules out the connector, and 2 different MAF's were used, so it is unlikely the actual MAF sensor that is the trouble.

PneumaticTire

No SES light? Did you try verifying the MAF power and burnoff relays have proper voltage? I know you said the relays are new but check there connectors and verify the have the proper voltage specs.

formula_pilot

No SES light and the sensor is getting steady 13+V power when the engine cuts out. Burnoff relay puts out 12V OK, all checked at the MAF connector .

86 Cali IROC

I saw another thread with the schematic of the MAF system. Maybe some one can pull it up. I get that same problem to a certain extent. However I do get the SES Light and it is the MAF sensor. I will just try to trace it myself.

Nails

found problem in my maf system. On mine (87) there is a
connector sitting on top of water pump and every time i
wiggled the wires engine would stumble or stall. I bought
a new pigtail from gm that bypassed that connector
and plugged directly into maf. the wires are about 12 inches long and splice into harness with shrink connectors.
set engine to stock specs and so far no probs and engine runs well. had changed to su-145 (bosch) also. hope that helps btw pigtail gm number is 15305932 and
ACD#PT1072 and it was over $100 cdn (ouch).

PneumaticTire

what connector did it go to? You mean it was a rubbed through wire?

Nails

it was the connector to the maf. on mine the factory harness there is a large connector on top the water pump, below the throttle body then it connects to the maf by the normal small connector. anyway when i wiggled the large
connector the engine would stumble or just stall out. so i
replaced that short harness with a longer one from gm which spliced directly into the harness with shrink
connectors. so it was a bad connection on the large connector and its ran great ever since. hope that helps.

formula_pilot

Thank you for the replys. I checked the connector and everything seems OK.

I know this does not seem to make any sense, but troublshooting indicates that there is an intermittent short in the Idle air control valve, which, in addition to causing IAC problems, is somehow causing a short in the MAF channel. I do not know much about the ECM internals, but it looks like the IAC short is definitely affecting the MAF channel.