Engine 91 5.0, Vacuum Line Connected to BAP Sensor??????

[sscobra]

Buddy of mine, picked of a 91 factory 5.0 coupe. He's been trying to get it to run correctly.

Previous owner installed a cam.

Here's the really weird part. While looking at things, happened to notice, they have a vacuum line running to the BAP sensor (like it's a speed density car). AhHa, that's got to be it! Buuuuut............When you unplug the vac line from the BAP(yes vac source was capped too), the car doesn't want to run. He's done all the basic stuff (base idle reset, checked for vac leaks, fuel pressure, etc., etc.).

Really lost on this one. Ideas???

Info:
91 Factory 5.0
MAF
A9L computer

Appreciate any and all help!

 

[AeroCoupe]

Have you guys verified all the other vacuum lines are correctly plumbed and in good shape? Pulled codes?

 

[sscobra]

Have you guys verified all the other vacuum lines are correctly plumbed and in good shape? Pulled codes?

According to him, all vac lines were checked. Codes were checked, no codes were found.

 

[Mustang5L5]

If it's an A9L, the vac line definitely shouldn't be connected.

Have you dumped codes?

 

[sscobra]

If it's an A9L, the vac line definitely shouldn't be connected.

Have you dumped codes?

Correct, we know it should not be connected. The perplexing part is, why does it not want to run when it is disconnected?

Dumped codes? We check for codes, there were none.

 

[Mustang5L5]

Correct, we know it should not be connected. The perplexing part is, why does it not want to run when it is disconnected?

Dumped codes? We check for codes, there were none.

Connecting it probably brings the expected values into a range where the ECU can make something of it and get the car to run. Disconnecting it likely puts the values so far out that it just can't run. The cam may or may not be playing a role here as well.


All i can speculate from the comfort of my desk. What's the quality of work by the previous owner? Looks well done, or do you see signs of other rigged stuff? How much vacuum does the cam provide at idle?

 

[sscobra]

Connecting it probably brings the expected values into a range where the ECU can make something of it and get the car to run. Disconnecting it likely puts the values so far out that it just can't run. The cam may or may not be playing a role here as well.


All i can speculate from the comfort of my desk. What's the quality of work by the previous owner? Looks well done, or do you see signs of other rigged stuff? How much vacuum does the cam provide at idle?

The work looks to be decent. Doesn't look like a hack job. Have to check on the vacuum at idle. The original symptom, was backfiring through the intake. Leads me to believe it could be timing related. Also, I too am trying to help diagnose from afar...lol.

 

[General karthief]

'no codes' means the computer is buggered, there are always codes ie; 11 is the code for system checks out ok, should at least get that.
We get the 'no codes' answer all the time, like 'it has fuel pressure ' cause fuel spurts out when the schrader valve is pushed.

 

[sscobra]

'no codes' means the computer is buggered, there are always codes ie; 11 is the code for system checks out ok, should at least get that.
We get the 'no codes' answer all the time, like 'it has fuel pressure ' cause fuel spurts out when the schrader valve is pushed.

Yes, that is correct. I too am at the mercy of the information I am given(like pulling teeth sometimes). I just verified, owner said 0 codes. I do believe, hey may have access to another computer to test.

 

[Mustang5L5]

Did the owner run the codes with the engine off AND the engine running?

Folks unfamiliar with OBD1/eec4 may not know that you have to pull codes in both conditions. It’s not like obd2.

The engine running codes would likely offer more substance.


Backfire in intake could also be vscuum issue/leaks.

 

[sscobra]

Problem solved!

Well guys, bad information strikes again. After originally asking what computer was in the car(was told A9L), turns out, when they went to replace it with a known good ECM, they pulled out a speed density ECM. It literally had "88" written on it with a marker. SMH!

Thanks for all the help!

 

[General karthief]

Not unusual for info relay confusion, I found when the shop scanner was used on my junk (89 GT) it tried to read it like a OBD2 and they had to imput something for 1 anyway, he may still have issues unless he puts the correct computer in. Considering it still has the MAF in place.
Makes you wonder what else is, well not correct.
Why not invite him to he can join the fun