04 S430 sudden limp home mode
 

Good morning. I have an 04 430 with 90k on it. Ran perfectly (except couldn't get trans to change to sport from comfort)until yesterday when I accelerated hard from a dead stop to enter traffic went maybe 30 yards and began to let off the accelerator a complete loss of power and the car went into limp home mode. I pulled off the highway. Read the engine code(s) and retrieved P0101 mass and a TPS/pedal position pending. Cleared stored codes and the check engine light is out. I can drive the car but there is no acceleration off the line but I can drive it if I accelerate slowly until it gains a little speed then accelerates normally and shifts correctly. It is not downshifting correctly and up shifts I have to hit the pedal more than. Quarter travel and then very quickly to get a down shift then most times it downshifts 2 gears. The check engine light is still off with these symptoms although it has lit 2 more times after erasing codes. I know this is a long post but want to get in all of my observations. Thanks

Bzoo, based on your post I would conclude you need a new mass air flow sensor. Other members may reach a different conclusion.

Thank you. I'm thinking the same thing

Monitor the MAF voltage directly from the MAF plug. If it is like the 97 era cars it will swing from 0 = no air flow to nearly 5 VDC at max flow.

Sometimes unplugging the MAF will result in a better running engine if the MAF is failing low.

Also, the 97 cars had 2 power feeds. A 12 VDC for the hot film circuit, and a 5 VDC to power the signal out circuit.

Thanks for the input so far guys. I disconnected the mass air flow sensor and the check engine light came on immediately of course but the drivability improved dramatically. Almost normal. However when I come to a stop sometimes the engine wants to stall like a standard shift with the clutch out when stopping. Even with the car in neutral. I'm concluding these are all mass air flow sensor symptoms. ..

No one has additional input?

i would first get some maf spray and see if that will help.... it could be a 10 dollar fix

When I owned my S430 this happened occasionally until I submitted and replaced the ESM (Electronic Selector Module). .02

Thank you both I tried the spray no luck but I should be cleaning my other vehicles so not a waste thank you. I will look into the selection module

You have 2 distinct problems here. One is the low engine power and the other switching from sport / comfort.

Fix the MAF problem first, it will affect shift speeds because the trans computer is expecting the engine to be working properly. Be sure the intake piping between the MAF and throttle body is intact, a opening here will cause MAF to read low.

If the trans is having a problem it will either stay in 2nd gear ( limp ) or the last gear selected ( until the key is cycled ) but you will have good engine power.

you mentioned that disconnecting the MAF improved drivability, this is pretty much a given for a failing MAF but some points are also to be considered - air leak at the throttle body etc.

the 4.3 litre V8 is like a 3.2 litre V6 with 2 added cylinders, these engines have a peculiar breather system which is plumbed from both valve covers and any leaks there also cause low MAF signal.

Whilst at it - perform a throttle pedal sync, It helped me in one very odd problem my car had, it would kickdown at even 30% throttle input and hold onto a gear even at full rpm redline which caused it to record a misfire code. The sync solved it.

It would also be wise to have your car scanned on a STAR diagnosis system and test the gear selector itself for all shift ranges and the S/W switch operation.

Sorry I just realized I had not tied this thread off...at least part of it...The MAS was bad...still looking to change the Selector module when I have a chance.

Thanks