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Low vacuum


NAXJA Member
San Diego, CA
1988 4.0 AT, 500 miles on rebuilt engine

This is a long one.

In an effort to squeeze as many mpgs out of the fresh rebuild, I've been trying to figure out why my vacuum is low at idle. Engine runs great, idles around I'd say 850 rpm.

When i started down this rabbit hole a couple of weeks ago, I was showing 17" of vacuum at idle. I'm at sea level so that seemed to me to be low for a fresh engine. I couldn't find any vacuum leaks with the usual cheap methods so I bought a smoke machine and found a few leaks: EGR pintle, MAP vacuum port at TB, temp sensor on the air box, and out of the top of IAT.

I capped the EGR, did the Cruiser mod to the MAP port, plugged the vacuum tree thing that fees the temp sensor and EGR, and gooped up the top of the IAT with some RTV. Aside from the goofy RTV job, the leaks all seem to be gone (tested a 2nd time with the smoke machine). However, my vacuum is even lower. LOL. Now floats a little bit right around 16" so I screwed something up somewhere.

So I went to work on the sensors: tested the O2 and it seems to be swinging voltage back and forth, MAP gave I think acceptable results when primed with a hand pump, IAT and CTS both tested within spec, and then I adjusted the TPS.

While adjusting the TPS, I checked the resistance on the ground and it was about 80 ohms. Thinking I may have found my problem, I started into running all new grounds straight to the battery (read that would work in another post or two). That didn't work, upon starting the motor it raced straight to about 3000 rpm. TPS was dead on and I did the Cruiser tip by adjusting the TPS while the engine was running. That worked, but upon start up it would race back to 3000. Slept on it.

I thought it might be worth a shot reworking the sensor grounds from the ECU rather than just sending them to the battery. That fixed the high rpm issue. Now all my sensors are seeing under 1 ohm at the ground, but I still have the same low vacuum at idle about 16"

I tested compression just to rule it out. All cylinders are 165-170 on a lukewarm engine.

One exception to the sensor regrounding, O2 sensor was not grounded at the ECU so I didn't do anything to that earlier. I checked that ground about an hour ago and it was about 20 ohms. I ran a ground wire straight to the battery for that one. Now showing around 1 ohm, but I haven't it started it since doing that. Kids had to eat.

I must be missing something somewhere. Any thoughts?
I noticed when testing the tranny plug on the TPS that I had no ground, circuit was open. I ran it to ECU sensor ground with the rest yesterday. Thought about it today and looked at the wiring diagram and reconnected to the TCM ground signal. Key on, still open circuit.

Pulled the connector at the TCM and tested from there to the TPS, under 1 ohm resistance, but something's not right. Shouldn't there I be able to test it from the battery negative to the TPS with key on?

Also, the yellow ignition wire into the TCM was clipped and butt connected to a different wire which runs over to the driver side. Looks like a TSB fix maybe. Cruiser, do you know anything about that? It looks like its bypassing the TCM fuse.
So I reconnected the TCM yellow ignition wire to see what would happen. I have no D on the transmission, but I can manually shift between 1 and 3, no 2nd.

I'll have to do some more digging on this. Any insight would be appreciated. This thread has gone off on a tangent I guess.
I should add that when I reconnected the yellow ignition wire, I did get ground at the TPS (trans side).

Really weird.

I put it back the way I found it since its more drivable.
My vacuum gauge may be a little off. I picked up a Harbor Freight gauge and it reads a little higher, an inch at most. So I'm back at 17" of vacuum about, it floats between 16.5 and 17.5.

I'm just going to track my gas mileage and see how it goes because I'm out of ideas.