Limp home - what's possible?

[tgreese]

My recent experience with my Golf prompted this question ...

My VW is a Motronic and has a MAF sensor. I've been chasing a recurring problem where it runs fine with no codes and then suddenly refuses to run at any slower than about 2500 RPM. It was throwing IAC out of range and Lambda out of range before I replaced the O2 sensor. Then all seemed to be well - I test drove with no codes. Turns out the MAF was going bad, but the computer trusts the MAF more than it trusts the IAC or Lambda, so it points to those first. On Sunday the MAF shorted and the engine hardly ran, so I limp home and park in the garage ... it's stumbling at idle, and I unplug the MAF, and suddenly I have a smooth idle and the engine revs from idle perfectly. The CEL is on, but the engine runs fine.

So this made me wonder, with the GM computer, how much can go wrong and it will still run? I know it has a limp-home mode, but what parts failures are guaranteed fatal and what failures can be limped on with?

[carnuck]

TPS is a biggie but contact cleaner will often make it limp home. On the VW, I'd unplug the MAF.

[tgreese]

I did, but then the CEL is on constantly, and I expect I'd never pass annual inspection. Plus mileage is said to drop without the MAF.

I bought a new Bosch MAF on Amazon for under $100 ... an excellent price, plus 2-day free shipping with Prime. Amazon has lots of useful parts on their site, if you can find them with the proper PN.

[FSJ Guy]

I have found that my GM system will NOT run will with a bad/disconnected MAP sensor. It does run, but horribly.

I haven't tried disconnecting the TPS. But a bad TPS would probably only give you idle. But at least at idle you'd still have power brakes and steering. You just wouldn't go uphill much.

[tgreese]

I'm curious about this. You'd think that a spare TPS and spare MAP sensor would be likely parts to carry along, if limp-home won't work without them.

[carnuck]

If the map sensor is totally dead then you have to read the diagram to see what one does what. Even a bad one may allow the motor to run a bit. If not then find which wire gets power for it to run like full throttle and split the resistance between idle and full throttle. (I know it's not worded right. I had to do this with my '86 Comanche in '00 when the MAP died bringing the truck home. Then I found later the vacuum line to it was cracked!

[FSJ Guy]

I've never had a MAP sensor fail. I've just unplugged it to see what would happen. : D

A spare TPS might not be a bad idea, but again, I've never had one fail. If you do carry a spare, make sure you have the correct Torx bit to replace it.