Hello all,
I've got a 1998 Monte 3.1 litre engine, 125,000mi.
I've had a P0401 code (exhaust gas recirculation insufficient detected) for a long time, but the car ran fine, until recently.
The car started missing on acceleration, a kind of stumbling, bucking, kind of feeling, up to around 35-40 mph, then smooths out and runs fine.
I pulled the throttle body and the passage from the EGR valve to the intake was plugged with carbon, so I cleaned it all out from the EGR valve through to the intake.
I was pretty sure I had fixed the problem.
I took it for a test drive and the car behaved the same, I've obviously missed the problem.
I get no MIL or trouble code on my reader.
No stored codes.
Emissions green across the reader.
Would anyone out there have any ideas about where to look to find this problem?
I'm baffled, and so are a few other mechanics
Any ideas are appreciated, can't give up on this car yet!
Thanks!
G.

 

When was the last time you did a tune up? Plugs? Wires? Could be the MAF sensor going out too.

 

Plugs 25,000 miles ago. Original plug wires tho' I don't remember the last time I changed the fuel filter. Hmmm maybe I'm answering my own question eh? Wouldn't the MAF throw a code?
Thanks

 

No, I've had a bad MAF before and not had a code. But I have seen them throw codes also.

 

How did you figure out the MAF was going or gone bad? Will it show up in an Ohm test?

 

Swapping with another one to be honest. You have to check the frequency of the MAF, even then it might not tell you if it's bad. If it was a consistent issue at idle, you could unplug the MAF to see if the motor would run better.

Start with your basic tune up and go from there. Never hurts to check your oil and antifreeze also - I've driven a 3100 with a bad lower intake gasket before and it likes to spit and sputter too... Milky oil is bad stuff.

 

Yep, I know about that, had it replaced a few miles ago. It idles so smooth I can't tell it's even running, and if I kick it hard it takes off around 3,000rpm. I found out the wife filled it up with 85oct. and the book says 87. I'm going to put new gas in it and see what happens. Due for an oil change too.
Wish I had some spare parts to swap around..

 

The only test you can conduct on the 3.1 is to check for correct circuit operation. Holding the connector in hand (with the lock clip on top) from left to right the prongs in the connector are #1, #2 and #3. The positive probe of your DMM goes to #3 and the ground probe goes to #2. With the key on you should have a reading of 12v. Also check your ground wires. If you don't have 12v you need to trace the wiring back to the distribution block looking for an open in the circuit.

The other test requires a Tech II (or equivalent) scanner so grams per second (flow rate) can be measured. A warm engine should read approx. 4 to 6 grams per second at idle. As the engine is accelerated the reading should smoothly and rapidly increase. If it doesn't rise or the rise is erratic the MAF needs replacement. Keep in mind though that the gram per second test requires a scanner.

 

Thanks, I did a voltage test on the MAF and did have 12 volts. I read about a test using the Hz setting on the DVM, and got a smooth reading as the I revved the engine. I've got a tech friend bringing his scan tool over to look at the readings. I also tested the resistance of the sparkplug wires, and got around 1K Ohms on each wire. I'm starting to wonder if I got some bad gas, although I don't get a misfire code. I changed the oil and filter and air filter, filled it up with the proper octane. It seems to be better, if I accelerate slowly. It also seems better when it's cold.

 

Your friend will be able to monitor more then just the MAF so the road to recovery sounds promising.