Hello Folks,

Having a strange issue. On two occasions I have had at hot start up, harsh shifts and a check engine light. Condition abates after a few minutes, and when I pull codes I have a stored code 126. On both occasions the truck was parked after being operated at speed, but nothing out of the ordinary, and no signs of over heating. Truck is in tip top shape otherwise. The MAP sensor was replaced last year and I am thinking that maybe I just have a faulty part.

The only other thing that comes to mind would be a heat soaked PIP creating problems, but I don't see how that correlates to the MAP sensor....

Your thoughts and suggestions appreciated.

 

Some folks have found their MAP sensor fills up with water due to the placement of the sensor along the hood/cowl line.

 

No water...

I wonder if the MAP sensor isn't heat soaked itself as there is a lot of heat in that area after the engine shuts down. Need a little fan to circulate air after shut down...

 

Band-aid fix....

If you want to spend money, purchase a meter that can read frequency. Diagnose the problem with the proper tools.

 

I was being humorous, but I can see that it isn't very apparent.

I am aware that the output can be tested, but the drive ability problem has proven difficult to replicate. I suppose it would tell me if the sensor is working properly otherwise.

Thank you for responding!

 

A faulty MAP output signal will cause drivability issues like you described. It's one of the main sensors the computer uses to calculate load, shift points, etc.

 

Update-

I replaced the MAP sensor with a warranty replacement soon after my original post and have so far had no further issues, but it has only been a few weeks. I did not test the output of the sensor as it seemed to be good odds it would be a waste of time to me with three basic outcomes. Tests faulty all the time = replace. Tests faulty sometimes = replace. Tests normal all the time = WTF.

If the condition returns I will update and dig further into the WTF possibilities.

Question - Is there a mechanical condition in the engine that could cause the vacuum signal to be off the wall at hot start up that would abate with a few minutes of operation?

 

Nothing comes to mind right now. Cold to hot perhaps a cracked manifold, gasket, etc. But nothing I can think of for a heat soak situation.

 

Well, went for a long drive today in the Texas heat and had a few things going on. After an hour on the highway the truck seemed to lack power at take off while I was on a feeder road. No hesitation, just slow, under powered. Seemed back to normal after being shut down for half an hour. On the way home back on the interstate I noticed the CEL was lit. Ran codes when I got home and there was a stored 172. Passed KOEO and KOER with no active codes.

So I think I have something leaking vacuum when very hot. I rechecked the accessory systems with a vacuum pump and I did not find any leaks while sitting in the drive way. No high idle, no hesitation. I am thinking maybe a heat related crack or gasket leak in the intake manifold, air injection pipe, or maybe the vacuum reservoir for the HVAC. Sticky EGR cause these symptoms?

Thoughts and suggestions appreciated.

 

A sticky EGR will typically trigger a CM code, but something to consider. Does the truck have two tanks? If not it could be a slowly failing fuel pump. Heats up then the output drops. Could explain the lean code and no issues with high idle. If you have two tanks and the problem happens on both, then the possibility of two going bad pumps is very low. In cases like this I drive with the fuel pressure tester taped to the windshield.

 

I re-checked the vacuum circuits and had a vacuum connector fall apart when I tugged on the line, so maybe the culprit...

It is a dual tank truck. Checked pressures this morning and both tanks check out - in the drive way. If symptoms persist I think putting a gauge on the windshield will be the next step.

 

Drive carefully....

 

What do you think about a bad ACT sensor or clogged catalytic converter as a possibility? Been driving for two days now and no apparent issues other than a sense of being under powered on take off when hot but I is hard to know if my butt dyno is accurate.

If it was the vacuum connector it would have been a minuscule leak I think, but I know any can be a problem. I was thinking of pressurizing the plenum to look for any more leaks, or using the propane method.

 

Code 126 came back and did not go away after cooling off. Replaced MAP sensor with cheapo aftermarket that I had laying around for years. Code gone, and running normally. Used vacuum gauge to check for restrictions, and all looks well. I am beginning to suspect a reference voltage spike or intermittent short as two new Ford sensors failing in their own in short order seems unlikely.

Seem reasonable?

 

Alright Folks,

Got out the multimeter this morning. Found no shorts. Grounds good. Reference voltage low at 4.9 volts. Pulled computer. One capacitor has begun leaking and damaging the board. Problem found me thinks.

Now to find a replacement...