Hot Start problem after MAF sensor replacement

Bherrington · 02-22-2021, 07:29 AM

2001 996 Cab with TIP, 61K miles. CEL came on but no change in performance. Now, though starter spins it nicely, it requires extensive cranking at WOT to start when hot, then pops and bangs through the exhaust before settling down to rough idle until revved to 2000-2500 for a moment to 'clear it's throat'. Starts nicely after sitting outside in 40 degree nights. Fuel economy reduced by about 1/3. Codes were 1126 and 1133, no misfires, upstream O2 sensors (only about 500 miles on them) cycled between about 0.15 and 0.95 volts. Tailpipes clean, so cats still working. No hiss when removing gas cap, engine stumbles when oil cap is removed; cannot blow through tube to EVAP valve but can easily suck gas fumes up through it. Cleaned MAF sensor, no change in problem. Replaced MAF sensor with the part for 01 cars -.#996-606-124-00 No change in problem.

Took car to dealer to have ECU re-flashed to work with the new sensor. Got car back un-fixed with an $11,000 estimate for 'needed services' with no comment about MAF sensor or ECU.

Turns out that the 'original' sensor was an early '02 unit - #986-606-125.01 no longer available. Understand that even the 02 cars won't work right with the new '02 sensor. Hate to burn out the cats by continuing to run car so rich, so now what?

Will the ECU 'learn' to work with the freshly installed '01 sensor, and if so when ? If I buy one of the new '02 sensors, will the ECU need re-flashing and how do I get that done if the local Dealer doesn't seem to know what I'm talking about ? December Pano not withstanding, this 996 has been nothing but trouble - if it had been my First Porsche it would definitely have been my Last Porsche.

JFP in PA · 02-22-2021, 08:33 AM

Quote:

Originally Posted by Bherrington

2001 996 Cab with TIP, 61K miles. CEL came on but no change in performance. Now, though starter spins it nicely, it requires extensive cranking at WOT to start when hot, then pops and bangs through the exhaust before settling down to rough idle until revved to 2000-2500 for a moment to 'clear it's throat'. Starts nicely after sitting outside in 40 degree nights. Fuel economy reduced by about 1/3. Codes were 1126 and 1133, no misfires, upstream O2 sensors (only about 500 miles on them) cycled between about 0.15 and 0.95 volts. Tailpipes clean, so cats still working. No hiss when removing gas cap, engine stumbles when oil cap is removed; cannot blow through tube to EVAP valve but can easily suck gas fumes up through it. Cleaned MAF sensor, no change in problem. Replaced MAF sensor with the part for 01 cars -.#996-606-124-00 No change in problem.

Took car to dealer to have ECU re-flashed to work with the new sensor. Got car back un-fixed with an $11,000 estimate for 'needed services' with no comment about MAF sensor or ECU.

Turns out that the 'original' sensor was an early '02 unit - #986-606-125.01 no longer available. Understand that even the 02 cars won't work right with the new '02 sensor. Hate to burn out the cats by continuing to run car so rich, so now what?

Will the ECU 'learn' to work with the freshly installed '01 sensor, and if so when ? If I buy one of the new '02 sensors, will the ECU need re-flashing and how do I get that done if the local Dealer doesn't seem to know what I'm talking about ? December Pano not withstanding, this 996 has been nothing but trouble - if it had been my First Porsche it would definitely have been my Last Porsche.

P1126 Oxygen Sensing Adaptation, Lower Load Range, Bank 1 – Above Limit
P1133 Oxygen Sensing Adaptation, Lower Load Range, Bank 2 – Above Limit
Fuel/air mixture so lean that sensing has reached its rich threshold. What that is saying is that DME can not make the mixture rich enough - you are running too lean (too much air to fuel ratio).

Potential causes are:
– Incorrect signal from MAF sensor
– Intake air system leaking
– Fuel pressure too low
– Volume supply of fuel pump too low
– Fuel injectors fouled

Bherrington · 02-22-2021, 11:49 AM

Thanks for the prompt reply. I'm an old Carb guy so FI systems are alien to me. Though you are the third who has said or implied weak fuel pump I do not understand why a lean mixture would make it start cold, but not hot, would make the fuel economy tank, would make it pop and bang out the exhaust after starting hot, and would allow the car to accelerate briskly to 85mph after 3 miles at 65 on a six percent upgrade ? Also if there is an air leak already, why would it stumble and run rough when the oil cap is removed, but not before ?

I think that your first suggestion is the key one, so how do I get the ECU to accept the signal from the newly installed, but different part number, MAF sensor ?