Great thought! I was going to diagnose the MAP, but a vacuum leak is starting to make more and more sense.

How much vacuum should there be at the valve cover inlet (oil filler cover)? At idle there is quite a bit of vacuum (will get inches tomorrow) pulling air into the crankcase. Wondering if the PCV is hosed? Where is that thing located on this engine?

 

5" inches of vacuum at idle? That's extremely low. With all of the symptoms you have I suspect multiple causes. Low vacuum reading makes me suspect severely retarded base timing. Common issue when someone is not aware of removing the SPOUT plug to set the base timing.

 

Full manifold vacuum is applied at idle to the PCV. Perfectly normal. PCV is located on the passenger side valve cover near the firewall.

 

Roger that, thanks... Somewhere under the plenum. Can't even see the back of tht valve cover.

 

Yeah, checking timing is on my list also. This truck has been messed with by multiple people, so just need to go through everything.

 

Thought about it some more last night and went out this AM to the shop to lean on the fender and think about this some more.

Tested all vacuum lines on manifold upper. All good. Hold vacuum. Could only get 10" on the brake booster with my hand pump, but I probably didn't have enough "pull" to overcome the internal workings.

Put the vacuum gauge right on the manifold. 5". Hmmm...

Check TPS, but it is running so poor now that I cannot get it to stay running long enough to test voltages. The TPS looks newer. It did run a little better with the TPS disconnected. Still weird.

Went to find the MAP sensor. it was on the fender by the starter solenoid. 0" vacuum on line going to MAP. Weird. Disconnected MAP sensor and truck started to run way better and could actually rev it up a bit (computer going into default mode?). Started to trace all the vacuum lines back. Rubber line on MAP goes to hardline and then the big vacuum connector after the relays. Tracing up by the EGR. Looks melted...

Sure enough, the bundle of vacuum lines got into the EGR line and melted them all. Pic below.

Subford was right on...big vacuum leak.

 

Great find!

 

Good job. Knowing how the system works makes diagnosing it simpler.

 

Update...

Got the vacuum lines spliced back together. Starts and idles better but only 15" vacuum now. At that manifold pressure the fuel pressure is 37-38. From the hand pump tests on the FPR, it needs about 20" to lower the fuel pressure to 33psi.

It idles much better now but still not right. Set the timing with SPOUT disconnected and it was off a few degrees. Made a little improvement in idle.

But...absolutely no power cannot move truck at all. Let clutch out and the load kills the engine. Try to rev it up and no go. Just a lot of moaning from the intake noise.

One weird indicator, when you try and open the throttle wide, smoke comes out of the valve cover air inlet. That seems weird like there should not be that much crankcase vapor at that RPM. Did plug off the brake booster again in case it was leaking but no change in condition. If you let the brake booster hose suck air the idle increases like it is leaning out like you would think.

Need to do more thinking on it. The one basic thing I have not done is a compression test 'cause I don't have a tester. Off to town later today to get one.

 

Check the TPS, it may be bad also.
You might want to open up the computer and see what it looks like inside also.

 

Update. Checked TPS and voltages are within specs. It is a 0.9 volts closed so could use a bit of adjustment later on. Resistance checks were OK closed and a little out of spec open. Haynes says 350 ohms WOT and I was getting 450. But the voltages on the WOT end were OK.

Did compression check. Getting 145-155. Clarification on those numbers. #1 was 145, but I had trouble getting the tester hose in the spark plug hole all the way. Suspect a bit of grime got in the threads. Will recheck that one when I can clean out the threads. It is a pretty blind location. Curiously #4 and #8 were 155. All the rest were 150. Do those numbers seem OK?

Going to test the MAP sensor tomorrow and dig out the computer and open that up.

Still wondering about why the fuel pressure leaks down so fast. Am starting to circle back on one or more bad injectors? Thoughts on that?

I am really learning a lot about these early EFI's, but this is just stumping me at every turn. Really appreciate all the help so far and all thoughts are appreciated!
Chris

 

I have a really dumb question, but did you replace the spark plugs? They're fouled up pretty badly from the sound of it and will cause all sorts of problems.

 

You know...I was looking at those this evening and wondering about that. Picture below. They are pretty black... A new set tomorrow is in order for sure. As I think these are shot? Thoughts? Not a lot of experience with spark plugs in a modern ignition system. Most of my experience is with plugs on 2 stroke snowmobiles. It was easy to foul a set out there and nothing would make them work again.

 

Those are toast. Use plain copper core Autolite plugs as a replacement.

 

Here's a concise guide to reading spark plugs for mixture. Your plugs are black but not wet so it's not likely you have leaking injectors you probably have several problems that are compounding the issues so be patient and work through them one at a time... you have already made progress.