Running rich after engine swap

[BikerGeek]

I love my truck, but if I can't work this out soon my wife wants me to sell it so I'm hoping someone here has some insight. The truck is a 1994 Mazda B4000 with a manual transmission.

I just completed swapping in a used engine to replace mine that had either a cracked head or block. (Long story there.) The engine came from a 1994 Explorer. It's a V6 4.0L w/EGR.

Since I put it back together, it shows the following symptoms:
- Runs *perfect* and smooth for the first 60 seconds after starting it.
- After 60 seconds, horrible hesitation, loss of power. Engine sounds like it's running underwater almost.
- A quick depression of the accelerator all the way down clears up the hesitation, but it returns 30-60 seconds later, even if I'm just maintaining speed.
- If I'm driving in 4th or 5th gear and it starts to 'bog down' a release of the accelerator, and then slightly depressing it to maintain velocity clears up the hesitation.
- Check engine light comes on and off. Stays on for around 2 minutes, then off for 10 minutes.
- Hooked it up and the code reader didn't report any codes. Also passed all tests.
- While running while hooked up to code system, O2 sensor readings don't adjust during acceleration. Constantly show same reading "0.98" (I don't know what that means, though).

This is what I've done so far:
- Checked all vacuum lines. They appear fine.
- Replaced both O2 sensors
- Replaced throttle position sensor
- Replaced all spark plugs and wires
- Ran a compression test:
Passenger side: 150 (all three cylinders)
Drivers side: 120 (all three cylinders)

I'm a computer geek by trade so I've reached the end of the things that I know to check (and probably have done some unnecessary checks/repairs).

Does anyone have any idea of where to go from here?

 

[jeremyjudd]

When you tested it did it have the check engine light on? The way these things are set-up there is no way that it isn't throwing codes if the light is on while you're checking it. You check all of your vaccuum lines? This is one of the least fun things to check/double check, but a lose vaccuum hose will make it run terrible and could explain all of the symptoms you explained above. Your EGR valve could be sticking too. Same symptoms. Does it run hot?

 

[BikerGeek]

Thanks so much for the reply.

I've checked them, but I am a novice so it's quite possible that I've missed something.

As for the running temperature, I only drive it 15 miles to and from work every day. It did start to get warm one day when I had to drive home for lunch and then back to the office. The temperature gauge moved about to the one o'clock position. Other than that one incident, it normally runs at about the ten-thirty position.

 

[Bird]

When you pulled the plugs to run the compression test, were the plugs on one side (passenger side for instance) all black and sooty while the other side all nice and clean ???

If so, then you have the O2 sensors reversed at the plugs - very easy to make that mistake and it'll do exactly what you're describing.

Bird

 

[UrbanRedneckKid]

Do either of the 2 trucks run California Emissions?

When I replaced my 94 cali 4.0 with a 93 fed 4.0. I had a problem with the same symptoms you describe.

I ended up having to use the ECU and the entire underhood harness from my donor truck to get it right.

My original motor used 7 wires in the injector harness being that it ran sequintial EFI (injectors fire independently)

My donor motor uses 3 wires in the injector harness being that it uses "batch fire" (3 injectors fire at once)