Well first I'd do a sanity check. While the car is running and you're connected to the ECM and datalogging, unplug the MAP sensor.
What happens?
Well first I'd do a sanity check. While the car is running and you're connected to the ECM and datalogging, unplug the MAP sensor.
What happens?
1990 Corvette (Manual)
1994 Corvette (Automatic)
1995 Corvette (Manual)
It will not stay running on its own right now. I have to keep it running by feathering the throttle. If I let off, it stalls within a second or two. Not enough time to get to the sensor and unplug it.
79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy
93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver
99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.
probably the map sensor is faulting and being overridden with a default value try resetting the ecm
That was one of the early things I did with no change.
I also tried another spare ecm I had with no change.
When I pick up a vac gauge this weekend, I'm gonna try and get some back probes and test the map sensor pins with the sensor plugged in.
79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy
93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver
99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.
doesn't take much to verify the wiring is good too, just backprobe the pins to the ECM
Well the parts store did not have a vac gauge or back probes.
However I feel like the issue is electrical and not mechanical, seeing as how it was intermittent at first. I may end up ordering a vac gauge online just to have one.
I did raid my wife's sewing kit for a needle and used that as a back probe.
I ended up not having anywhere near as much time this weekend as I thought I was going to have.
With the sensor plugged up and the ignition on (engine not running), I have 5v at the power wire, and have 4.88v on the sensor out. The voltage drops as I apply vac to the sensor.
I then go to the ecu and check the pin there. I have the same voltage readings (ignition on, engine off).
So it would appear it is making it to the ecu. However I did not have the laptop hooked up.
I took loose the distributor and gave it about one degree of a turn just to see what would happen.
I got it to fire off and run. Still ran like crap and missfired like crazy, but after about 30-60 seconds, I got it to idle on its own (still running poorly).
While it was running, I was feeling around the wiring and just looking things over and it suddenly stumbled badly and died.
I am starting to think I have a bad wire, that is somehow testing good, but for some reason is not getting the signal to the ecu. Or is showing good when I test it, but is not when the engine is running (for some reason)
I have some spare pigtails and terminals for the weatherpack connector for the map sensor, but I do not have any spare pins or pigtails for the ecu connectors. I do not really want to cut the wire close to the ecu to run a new wire, only to find it is that that wire. I need to find some pins and make a temporary new wire from the map signal to the ecu and test it.
79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy
93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver
99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.
Maybe this connector kit from Casper Electronics will be helpful? http://www.casperselectronics.com/ca...product_id=552
The kit has MAP pins and TBI ECM / PCM Pins
Last edited by dave w; 07-24-2023 at 04:37 AM.
Hey, nice to see you here.
MAP sensor high indicates the ecm is reading high voltage on the MAP signal line during specific operating conditions. For default configuration, here's the code definition:
33 MAP sensor signal voltage was too high (low vacuum) for 1 second when throttle opening was under 20% and engine speed was above 1250 RPM.
You should be able to see MAP voltage on the scantool in key on / engine off condition to compare the reading obtained at the PCM wiring harness connector using a DVOM. Additionally, you can create simple resistor networks to connect between 5V, signal, and ground to create a "between" voltage to send to the pcm.
In a stock wiring harness the 5V circuit and GND circuit for MAP are shared with the EGR valve. Any issue which can cause a short to 12V on GND or 5V REF or an open on the ground circuit will also raise voltage on the signal line as seen by the ECM. To catch this on the DVOM you should be measuring voltage between the sensor ground and the sensor signal line.
Since the engine reacts badly while wiggling the harness, I would likely try connecting the laptop to watch ALDL data then taking hold of the harness and performing a Ford "Wiggle Test." Watch for significant changes in MAP or another sensor reading on the laptop.
I have had troubles years ago with intermittent code 33 on TBI equipped 1995 vans. In one case I found a broken wire in the harness causing an intermittent open. It was only apparent when the engine was very warm. In another case an aftermarket EGR valve caused an issue. In both cases it was frustrating to find because there were no symptoms showing while the van was in the shop.
Keep us updated!
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