Pontiac Repair: Crack in intake manifold, pontiac grand prix, oxygen sensors

Question
Kris,

I have posted a couple of questions regarding my 2001 Pontiac Grand Prix. It won't let me ask another follow up question so I am doing it new. The original questions and your answers are below, but here is what I have found out:  I finally took the car to another shop here and they told me there is a crack in my intake manifold and it needs to be replaced. Do you think it make sense that an intake manifold crack would set P0101 and P0300 (and perhaps the p0108) codes? It seems to me that if the crack is in the manifold it would have already bypassed the MAF sensor and that the sensor should no longer be a part of the equation. Thanks for your help!

Nate

<original correspondence below>
Kris,

Sorry if you get this e-mail twice. I ran the codes on the car and it had 4 of them. P0101 "Mass or Volume Air Flow Circuit Range/Performance" and P0300 "Random/Multiple Cylinder Misfire Detected" (this one I assume is a side-effect of my problem, not the cause of it). The other two codes were the exact same numbers only they had the letters 'p' & 'd' stacked on top of each other after the number.

We reset the codes and took it for a test. Afterwards we rechecked the codes and it had 3--the same two from before plus a new one "P0108 MAP/BARO Circuit High Input".

Its looking very much like an air flow problem, but I can't put my finger on it and I'm sort of at a dead-end with it. Can you offer any suggestions as to what I might try? I'm still not thinking the cat is the problem because that seems like it would give me bad readings on the oxygen sensors if air wasn't getting through it. Thanks in advance for any help/advice. You guys are awesome.

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Followup To
Question -
Kris,
Thank you for providing the assistance that you guys do here. All of your responses seem to be genuine and I know you guys really know you're stuff. I hope you can help me with a problem I am having with my 2001 Grand Prix GT.

A few weeks ago I started having serious problems accelerating. I would push the gas down and it wouldn't take off. It would eventually accelerate to speed but something was obviously wrong. Thinking it could be a transmission problem I took it to AAmco for diagnostics.

Aamco concluded that it was not a transmission problem and that the problem was in my spark plugs and catalytic convertor. The said that the codes indicated "multiple misfires" and that had caused my cat to essentially meltdown and clog up. The repairs they quoted me (including their 'necessary' extras such as fuel injection cleaning and transmission flush/fill) came to a shade under $1,400 (including a quoted price of $400+ for the cat alone, not included installation, what a ripoff!).

I changed the plugs myself and tested the wires for resistivity (they were all less than 10 ohms and seemed good) but this did not help. My car will now occasionally "take off" when I let off the brake and accelerate on its own up to about 25 MPH--completely different problem than I was having before. Also, sometimes when I am stopped the RPMs will repeatedly "jerk" up and fall back down to normal.

Does this seem like "just a clogged catalytic convertor" to you? I plan on taking the cat off and seeing if that corrects my problem this weekend. If it does, I will just replace the cat but I have a feeling this will not solve my problem. Any help you can offer will be greatly appreciated! Thanks!

--Nate
Answer -
Thanks Nate, we try our best. Your instincts about a plugged cat are spot on. If you had an exhaust restriction then it would be very rough running, have no power and would die constantly. You said they pulled codes but is your check engine light currently on? This is the first plan of action. Have a competent shop hook up a scan tool and not only check the codes but also look at the datastream to see if there is a sensor blatently out of range. My first guesses would be the throtle position sensor, idle air control motor, temp sensor, or oxygen sensors. With the complexity of modern computer controled engines you really have to have the datastream information long before you start replacing parts. Hope this is a little help for you and please send me back a progress report with some info from the scan tool. Good luck :)



Answer
I would deffinately go along with a bad MAF sensor. It would explain your hesitation and hunting idle problem. As far as the MAP check to make sure the vacuum line is ok going to it because these rarely go bad. And yes, the misfire is probably an effect from the air flow problem. Great detective work...you didn't need me after all :)

Answer
Yes a crack in the intake would set off those codes, the intake is dry so it is air only and anything that causes air volume and vacuum to be out of the preprogrammed ranges will start to set codes. Because the computer can't think of physical damage causing out of range limits it will extrapolate the info available and determine which sensors might possily cause the condition it is seeing. With a loss of vacuum the map will be off and the extra air coming into the system isn't being properly balanced with fuel so you get an out of range on the maf which it can determine by oxygen sensors and the incorrect air/fuel ratio causes the misfires.