Good troubleshooting. VREF at 4.9 VDC is not unheard of, but it did lead you to open the computer.

 

Re manufactured computer installed. Started on first crank and idled until warm. Shut down and ran KOEO and got 111 and 111. Cranked up and ran KOER and got 111. Went for a 10 mile drive of mixed conditions. No CEL or drivability problems.

Impressions from first drive. Crisp throttle response and shifts, like a new vehicle. No loss of power or responsiveness after highway driving. More stable idle than before. Considering I didn't have any complaints before the MAP sensor problems the improvements are pretty sweet. It is amazing how degrading performance can slip up on you.

I will keeping an eye on things for the next few weeks but hopefully I won't be back down the diagnostic rabbit hole for a while.

Thanks for all your help Randy.

 

Glad to see you are on the the right track. Thanks for the follow-up.

You are welcome

 

Well, we have a problem.

First start up yesterday - lots of cranking but no starting. Removed key, took a breather, and tried again. Repeat. Repeat and then start. Cycled on and off maybe a dozen times with no further trouble. No problems the rest of the day, with lots of experimental cycling on and off.

Second start up today - no start on first crank. Start on second crank like normal.

So me thinks there is a problem. Bad timer circuit in new computer? Sticky relay? Weak connection?

Where to start on troubleshooting?

 

Exchange that "rebuilt" computer. If you want to keep your sanity intact recheck for spark and the fuel pump is getting energized. If you dare to open that rebuild computer you will probably find very poor repair job or none at all. Be aware opening the computer up will most likely void the warranty.

 

I knew it was a risk that the re manufactured computer could be bad, but it is still freaking aggravating. It is the loss of time that bothers me more than the money or effort, but there is nothing else to be done.

Warranty replacement on the way as is another unit from a different retailer. Neither will be here till Monday... so I'll be parking the truck until then.

 

Update.

2 weeks with second computer installed and no problems so far.

Thank you for your help Randy!

 

Glad to see it looks like you got this one whipped

 

Well I'm back... Had about a month of trouble free operation.

Last couple of hot starts engine surges and then almost dies, then level offs and drives like normal. Cold start up has been normal.

Then on the way home this afternoon the CEL came on along with harsh shifts. First CEL I've seen since I installed the replacement computer.

KOEO, 126 in active and 126 and 172 in stored memory.

It still seems like the MAP failure is heat related, but that just doesn't make a lot of sense to me. Does explain the surge and hesitation at hot start up, And I suspect the intermittent MAP may be causing the lean code but I don't really know.

Is there a link between the two sensors I am not seeing?

Your thoughts and suggestions appreciated.

 

I would start looking at the wiring between the MAP and computer. Yes a faulty MAP signal can trigger a lean or a rich condition. It is one of the main sensors the computer uses to calculate how much fuel to supply and what the load is for transmission shifting.

 

Do the wiring check for sure, but I gotta' ask, what brand replacement MAP?

I went through a couple Borg Warner (lifetime warranty), or whatever they are before grabbing a used Motorcraft at the yard.

 

New Motorcraft sensors every time. I have an old house brand unit that I bought for a different truck 10 years ago that I use until the next replacement comes in. It makes me wonder if there is something wrong in the latest batch of Motorcraft MAP sensors that is making them prone to heat damage. I think something being wrong in with the harness is more likely, but I still feel like I am chasing my tail some times.

I checked continuity and grounds on those circuits already, but I think I need to pull the harness and do a careful visual inspection as well as rechecking my previous work. It should run down the passenger side fender and across the radiator support correct separate from the rest of the engine control harness, yes?

 

The direct route would be across the firewall then down to the PCM. The connector at the MAP is C1011. The Signal Return is done via Splice S137. VREF is another splice S163. Both are located near the power brake booster/PCM. Seems odd if it did a complete u-turn around the radiator support.

 

Okay, did some poking around after work.

So, the Oxygen Sensor and the MAP Sensor harnesses are routed from the EEC down the fender, across the radiator support, and up the passenger fender splitting up near the starter relay. They are bundled in with the lighting harness. There wasn't any apparent damage to the harness or wires after a preliminary visual inspection in place. I will need have to more time available to be able to pull the whole harness out to really check it out.

I disconnected the harness on both ends and checked continuity and grounds again, and all is well there. I also checked for high resistance and did not find anything. The Reference Voltage and Signal Return circuits show shorts to ground, but I assume that is normal as there are other grounded components in the circuits. Is this incorrect? The MAP signal circuit shows no short to ground. I presume the only way to check for a wire to wire short will be tap every wire sharing the bundle? I am more suspicious now of there being a problem in harness as the two troublesome sensors are the only two routed in that harness.

Is there anything else I should be considering? Simply a bad sensor out of the box? I would think a voltage problem from the battery or alternator would cause problems across the board...

 

Signal Return is essentially a stand alone circuit to ground, it should measure very low resistance/short to ground (it is ground via the computer). VREF on the other hand, should not have the same characteristics. If it is measuring a short to ground you have a serious problem affecting several other sensors. TPS, ECT, ACT/IAT, EVP rely on that VREF signal.