Vacuum mis-reporting to ecm
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4wd Low
Joined: Apr 2023
Posts: 10
Likes: 1
Vacuum mis-reporting to ecm
I have a 91 F150 5.0 E4OD that had a blown head gasket a while back. I have it put back together and running but I get a stumble at idle and fuel economy is bad. I used a Bosch 1300 to pull live data and this is one frame of what I got.
Record Data
FRAME: -4 TM: -0.5
CANST PURGE On
ECT(V) 0.6
ECT(°F) 189
ENGINE SPD 952
EVP(V) 0.4
EVR(%) 0
FUELPW1(mS) 6.0
IAC(%) 53
IAT(V) 1.3
IAT(°F) 144
LOOP STATUS Open
MAP("HG) 10.6
O2SEN 1(V) 0.87
SPARK(°) 15
ST FTRM1(%) 8
TP MODE C/T
TPS(V) 1.0
VPWR(V) 14.0
VREF(V) 5.0
VSS(MPH) 0
I had a vacuum gauge hooked up at the that was sitting at about 20 inhg at the same time I took the data I have 5.0V ref at the sensor and the frequency is pretty close to a chart I found when I use a vacuum pump I have even tried 2 different ECM and 3 different MAP sensors nothing changes. I did note in the Ford shop schematics it shows a potentiometer and I have 5.0 V on the sensor wire going to the ecm with no sensor plugged in. next I'm going to trace the wires from the ecm to the sensor to make sure nothing has failed there. Has anyone seen this before?
Record Data
FRAME: -4 TM: -0.5
CANST PURGE On
ECT(V) 0.6
ECT(°F) 189
ENGINE SPD 952
EVP(V) 0.4
EVR(%) 0
FUELPW1(mS) 6.0
IAC(%) 53
IAT(V) 1.3
IAT(°F) 144
LOOP STATUS Open
MAP("HG) 10.6
O2SEN 1(V) 0.87
SPARK(°) 15
ST FTRM1(%) 8
TP MODE C/T
TPS(V) 1.0
VPWR(V) 14.0
VREF(V) 5.0
VSS(MPH) 0
I had a vacuum gauge hooked up at the that was sitting at about 20 inhg at the same time I took the data I have 5.0V ref at the sensor and the frequency is pretty close to a chart I found when I use a vacuum pump I have even tried 2 different ECM and 3 different MAP sensors nothing changes. I did note in the Ford shop schematics it shows a potentiometer and I have 5.0 V on the sensor wire going to the ecm with no sensor plugged in. next I'm going to trace the wires from the ecm to the sensor to make sure nothing has failed there. Has anyone seen this before?
Logistics Pro
Joined: Aug 2014
Posts: 3,558
Likes: 852
I'm dealing with a hard cold start issue, and a whistle noise during high idle and load, and slight stumble once in a blue moon.
Whistle noise is definitely a vacuum leak.
Finally, yesterday found the culprit. The EGR valve is leaking/sucking air. Going to replace that today.
Bad EGR can give you poor economy.
Whistle noise is definitely a vacuum leak.
Finally, yesterday found the culprit. The EGR valve is leaking/sucking air. Going to replace that today.
Bad EGR can give you poor economy.
Laughing Gas
Joined: Oct 2014
Posts: 946
Likes: 186
From: Carnation, Wah
No help with the MAP sensor- although I've had similar problems-
but you're also open- loop with a coolant temp of 187f. How's your oxygen sensor?
The ECU would love to use it to lean the engine out, as the base
(open loop) maps are pretty rich.
fwiw,
t
but you're also open- loop with a coolant temp of 187f. How's your oxygen sensor?
The ECU would love to use it to lean the engine out, as the base
(open loop) maps are pretty rich.
fwiw,
t
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
Yeah you have to figure out why the PCM is still in open loop at that operating temp, idle speed is also high(should be 650-700) and closed TPS should be under 1v... TB set screw may have been tampered with
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4wd Low
Joined: Apr 2023
Posts: 10
Likes: 1
Hi Paul, Thank you for the reply. I need to reset the TB I drove it around the block after figuring out that a transistor in the ecm was blown and causing four of my injectors to fail. This was initially caused by the insulation on three of the injectors getting hard and crumbling from age and shorting the additional current was too much for it. What temp should the loop close? I did pull that data shortly after a restart. I had shut it down to check some vacuum lines and the EGR valve and when I restarted it it started to rain so I just pulled the data and closed up for the evening. I'll get the idle in line tomorrow then check the EGR position sensor and control valve. Maybe the high idle is why the ECM can't identify the cylinders during the KOER test?
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4wd Low
Joined: Apr 2023
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Thanks for the input! I was thinking the map signal should jive with the gauge.
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4wd Low
Joined: Apr 2023
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I just changed it but haven't pulled the data since. I was concerned it got contaminated when I had to put some head gasket sealer in to limp it home.
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