Hi all, please see below and let me know your opinions:

Car has intermittent stumble < 2500 rpms. Can't seem to isolate it yet. Some days stumbles, other days runs smooth. Here's what I've done so far:

I've checked fuel pressure - ok per Haynes and other posts on this board
Coil wire was corroded - replaced with new A/C Delco
Replaced all other ign wires as well: A/C Delco
Replaced coil A/C Delco
Did spacer mod for coil/ICM
Pulled a plug and it looked clean. Plugs A/C Delco platinum have 15k miles on them
Cleaned MAF element
Replaced torn intake elbow
Replaced fuel filter
Checked for codes using a new Innova OBD1 reader: Nothing
Battery voltage: 12.5v engine off, 13.9 to 14.1v motor running

MAP voltage (this is the part that I'm unclear about): Used a digital multimeter probing green and black wires:

KOEO 4.7 v
Engine running idle: 1.19 - 1.2 v
Engine running WOT: 0.96 V

Clearly something is off with the MAP sensor. Volts should have climbed to 4.5 at WOT, but instead it was only 0.96??

Should I relpace the MAP sensor? Could this be the part that's causing the elusive stumble? How about the battery? Is it putting out enough volts? Haynes says 14 -15 v with the motor running is normal. It's an Exide Silver battery 2.5 yrs old.

Any help is appreciated.

 

My map on my datalog shows .25 at idle and when i go wot it reads what the baro shows which is 1.04 from my location. I believe you are looking for reading that are different than mine. I believe you test the map with the key on but not started and then start the car and it should go up. You should get a code if it really far off. I know a loose connection or broken one like mine was can give inaccurate readings. Maps seldom go bad but it happens. Mine was only slightly off and i replaced the sensor, connector and the gasket for $65. I have no problems or weird reading anymore.

 

When you go WOT with the engine under load, the MAP reading should approach the 4.7V (barometric pressure) you get when you first turn the key to on, but don't start the engine. A faulty MAP reading will have a major impact on how the engine responds to throttle transitions, since the MAP reading is the basis for where the PCM sets the ignition timing, and which long term fuel correction cell it uses.

You are reading volts, mrmint69 appears to be reading a range of 25kPa to 104kPa. Both are correct, although 25kPa seems a bit on the low side (~22.5"Hg vacuum).

The culprit is often a broken harness connector on the MAP. They dry out and start to crumble, providing erratic MAP readings.

 

I bought the connector thru the dealership and it said AC Delco on the package. It appears to be no different then the $12 ones on Ebay. The orange gasket thats $3, if it is bad could cause problems but i am sure thats not often.

 

thanks brent and mrmint. how can i isolate if its the harness or the MAP itself? both the clip and the pigtail are broken on mine. what about the battery voltage? is mine normal? i've read a few guys here had low rpm stumble and then they got a new batt, the stumble went away.

 

I would just replace the connector. Ebay has them cheap and i got mine from the dealer for about double that. My battery voltage on my datalog is in the 12's also. From 12.3-12.7 so yours sounds similar. The computer gets it readings but it not very accurate. Check the battery directly to be sure. I went to the auto store and they said my readings were a little low but everything checked out fine.

 

just ordered the connector on ebay 14.00 shipped. how do i replace/join it with the old wires though? do i splice the old wires and join the new with electrical tape or something?

 

If you want to do it right get some solder and some heat shrink tubing.

 

That would just about guarantee you trouble. Solder and heatshrink.

 

The Ac Delco connector came with the crimp style and it worked good and i used some electrical tape as well to make it water tight. I assume you were talking about this and not just using electrical tape alone.

 

i'll have to get some crimps, and tape em' up too. unless the ebay ones come w/crimps??

 

Update: I replaced the green MAP harness and soldered wires etc. Checked the voltage on the new harness and it has not changed. Same as in original post.

KOEO 4.7 v
Engine running idle: 1.19 - 1.2 v
Engine running WOT: 0.96 V

Also, changed out the battery to a new A/C Delco, car still stumbles.

1) Does the MAP need replacing?
2) Could the TPS be causing this?
3) Could the Granatelli MAFS be causing this?
4) Could the crane 1.6 RR'S be causing this?

Thanks in advance.

 

[QUOTE=3) Could the Granatelli MAFS be causing this?[/QUOTE]

Yes! I had all kind of issues before i changed mine and my car runs better and i cut a nice chunk off my 0-60 times after replacing it. They appear to be a problem more often than not. I got the AC Delco on Ebay for only $60.

 

Hmm...very interesting about the Granatelli. Wonder if that's whats causing my MAP voltage to be low????

 

The Granatelli MAF is not causing the MAP sensor to read 0.96V at WOT. Sounds like you need to replace the MAP sensor. Have you taken the time to remove the MAP sensor, and check the rubber seal, and make sure the tube that goes into the manifold isn't filled with oil?