MAP sensor question

[Chuck Norris]

Hey guys! Having an issue with bucking in my 2004 CVPI. I have access to an app that has some scan tool info, just basic stuff. I was able to read the pressure on my MAP sensor, with the engine off, key on. It read 9 inHg. My local barometric pressure is about 30 inHg. I read online that they should be equal if the car is not running. Could this indicate a bad MAP sensor? Now, I know you might be saying "Chuck, you goofball, these cars don't even HAVE a MAP sensor!" Well, yes and no. They have TWO, actually, built into the EGR module. Yeah, surprised me too. Anyway, the only issues the car is having are lackluster acceleration and bucking at low RPM. Idles at about 850, with some possible light surging, but it's hard to say for sure. Car is so quiet it's hard to say.

If you have any info for me, I'd appreciate it. I understand the EGR DPFE is a weak link on these cars, but the EGR module is about $120 the cheapest place I could find it. I'm not getting any codes, of course, because that would be too easy.

[sly]

Check the MAF sensor and make sure it's not dirty. MAF or Contact cleaner would sort that. Also, use your app to check the ECT, CHT (cylinder head temp), and air temp sensors and make sure those are sane. The EGR usually isn't an issue on these as much as said DPFE sensor. Stuff may need cleaned though. If it's not throwing a code for the EGR stuff, it's probably fine. Check the temp sensors and if they're sane, and the MAF is clean, unplug the MAF and see if the car dies or keeps running the same. If no change, replace the MAF. If it dies, replace the baro sensor.

[gadget73]

i suppose a DPFE could function as a MAP, though it would only work on one side, and only if the EGR valve was shut. The other side should see positive pressure from the exhaust. Not sure the ECM uses that info to control how the engine runs though, its only looking for a pressure drop across the egr valve. Basically it flows through a choke point and it uses the drop across it to gauge how much flow is going on. I suppose if one was wonky it might screw with how far the egr valve opens though since it might miscalculate the flow.

anyways, are the hoses to the DPFE itself in good shape? Sometimes they melt or clog and that causes stupid problems.

[Chuck Norris]

I'm going to check the hoses. When I first unplugged the MAF, it changed nothing during the first drive cycle. During the next drive cycle, MAF unplugged still, it drove better than ever. MAF is clean, and the app let's me check AIR and ECT levels, and both looked right. Temp here is about 90 during the day, and IAT was within a few degrees. ECT matched the dash, essentially. I will check the hoses. What I was saying about the GET being a module - the DPFE is a permanent part of the EGR, not replaceable separately. So is some other gizmo, which I assume is the MAP sending unit. So if DPFE goes, it's $120 worth of junk. I will check all hoses - but what do you guys think about the difference in barometric pressure?

[Chuck Norris]

I meant to say it lets me check IAT, not AIR. I got autocorrected.

[sly]

If unplugging the MAF changes nothing or it works better... that's most likely your problem.

[gadget73]

yeah, thats the usual diagnostic process for a dud MAF. If it works better without it, its NFG. If you haven't flushed it off, try that first though.