'87 Colony Park, stutters, bucks, and stalls. Help Pls!

[jeff97062]

I disconnected MAP hose and it died. MAP was definitely replaced also. Doesn’t mean it isn’t failing, just stating it as a fact after inspection.

[jaywish]

The opposite of a backfire. Small engine explosions into the intake.

[jaywish]

When installing wires make sure they particular ones are crossed as shown if you look very closely at diagrams.

[sly]

Looks like the MAP sensor is fine. It did what it should have done. We can probably rule it out as an issue.

[gadget73]

yeah if it didn't magically start running like normal, the MAP isn't dead. Normally when those are unplugged the fuel goes super rich and usually it dies, or at least runs completely horrible.
They rarely fail in some sort of not-dead manner. There is a way to check it with a tachometer to verify frequency output is within spec but if its already been replaced and didn't change how it runs, I'll vote "not it".

personally I'm not much for throwing parts at a problem without diagnostics, but it sounds like thats what has happened here. At this point its hard to know if the original problem remains or if there are new problems introduced.

[jeff97062]

I replaced the vacuum tube into the MAP, to no difference. but then I got out the multimeter to check wiring to the MAP. It's got 7 volts going to it, when I believe it should have 5. The rating of ground to signal is supposed to be 1.4 to 1.8....and it's 2.05. Could bad wiring be the source of the issue? My fuel injection was rewired by the shop because mice ate the wires through them. When they replaced that, is it possible that jacked up my wiring to the point it did this? I have a new motorcraft MAP coming in tomorrow, in hopes the Duralast MAP they put in is just crap....but not convinced it will solve anything.

[gadget73]

what were you using for a ground connection?

[jaywish]

The map test you did indicated probably good as sly and Gadget mentioned.

Did you check the fire order?

The injector wiring could also be an issue but first check the fire order.

BTW could you send a pic or two of the engine?

Also does it pop back through the intake?

[jeff97062]

Yes, I checked firing order is SO 5.0. I tried both(HO and SO) to be sure even, but SO works.

See pics, as you requested.

Lastly, how would I check to see if it’s firing back thru the manifold?

[jeff97062]

having trouble posting image...

[jeff97062]

http://www.grandmarq.net/vb/album.ph...chmentid=58103

http://www.grandmarq.net/vb/album.ph...chmentid=58102

http://www.grandmarq.net/vb/album.ph...chmentid=58101

[jaywish]

You would hear it popping in the intake

[gadget73]

I'm still just curious where you were getting 2 volts on the ground circuit. That may actually be the problem, depending on where exactly that measurement was across.

There is a ground meant to land at the rear of the lower intake manifold. It comes out of the harness that feeds down to the oxygen sensors. Thats the main engine sensor ground return wire. If that is off, all of the sensors end up skewed by whatever the voltage difference is between the engine and the battery. That shouldn't be more than about a quarter of a volt, but whatever it is, its enough to cause issues. If your cables are funky enough to cause 2v of difference between the circuits that may be the whole problem.


another dumb thing that can happen, if someone has swapped the oxygen sensors and got the harness out of place, its entirely possible to connect the right sensor to the left connector and vise-versa. It makes the engine run really stupid when warmed up, but should be basically normal when cold. The connectors are identical, the only real indicator is the wire color. I don't have a manual in front of me to tell you which is which, and you'll need to peel back the tape a bit to figure that out anyway. Fair bet every wire visible next to the plug is some tan-like color instead of whatever it ought to be.

[jeff97062]

Thank you gadget73! Will take a look at the ground there