OBD I Code 27 diagnosis, please help

Ok, so I changed the injectors in my 1994 Country, 4.0, Auto trans about 8 months ago. Aside from a resulting engine compartment fire :shhh:, the swap went great, and the Jeep has been running flawlessly. With gas prices as high as they've been, the Jeep kind of got left to the wayside for the last couple months (2-3).

I started it up yesterday, it smelled kinda funky but was idling nice. I figured the odor was just build up of crap from not being run in so long. I drove down the block to grab some coffee and come home. By the time I got home the jeep was idling funny and I was getting a check engine light.

Being '94 it only has OBD 1 and was giving error codes of 12, 27, 55

I just changed the battery before I started it, so that explains the 12, and 55 is the end of the codes, so it explains itself, but the 27 has me stumped.

From JeepHorizons.com I got the following:

27 Injector circuit isn't switching when it's told to (TBI)
OR (MPI) injector circuit #1 not switching right
OR (turbo) injector circuit #2 not switching right
OR (all 1990-) injector output driver not responding
- check computer, connections

From JeepForum.com I got the following:

27 Logic module fuel circuit internal problem or TBI injector does not respond properly to control signal

Any suggestions on the best way to diagnose this? I find it hard to believe an injector went bad from just sitting around for 2 months. I certainly don't want to rush right out buy a new PCM if that's not it. Is there another computer box along the way I need to worry about?

Could it be the result of a faulty tps or cps?

Any help is greatly appreciated.

Michael

P.S. I guess I should also mention, when revs hit about 3500 it feels like all 6 cylinders start firing again. Right now it sounds and feels like it's missing 1....although the power and roughness of idle seem to come and go. If I let it cool down, the problem isn't as noticeable. Today I jumped on the freeway and went about 45 miles with no problems and no CEL. As soon as I slowed for an immigration checkpoint, the miss started again and never went away....letting the engine cool back down now to see if it starts right up or if it's gone until it gets warm again.

Your 94 is multi-port injection so "(MPI) injector circuit #1 not switching right" is the error. Check the injector rail wiring maybe? :dunno:

It's normal to have the miss seem to disappear at higher rpm's. It is still probably a non-firing injector.

You can do a very quick check to determine whether the problem is in the injector or the harness, by running the engine, and unplugging the injector you suspect is bad. If it's a bad one, nothing will change. If it's a good one, the engine will briefly bog, then recover. Now swap the connector that tested bad with its nearest neighbor, start the engine again, and repeat the test. If the same plug is dead, it's the harness. If the same injector is dead, it's the injector.

Check voltage at the injector plug, and resistance of both positive and negative lines. This system switches the negative side, and positive is constant-on with ignition, supplied via a line with branching splices to each injector plug. The splices can go bad and give you low voltage at the injector. You should check resistance of the pos. line back to the connector at the coil, and the resistance of the neg. line back to the PCU.

Injector 1 goes to pin 16 on the PCU. #2 goes to pin 15, 3 to 14, 4 to 13, 5 to 38 and 6 to 58. The positive line to all injectors is dark green/orange, and traces back to the ASD relay, as well as one of the pins on the coil/distributor plug. With ignition on, engine off, you should get a full 12 volts between the hot terminal on the injector plug and ground. If it's a bad splice, it might start out relatively good when cold, and deteriorate as it heats up. If you have a bad wire or splice in the harness, it's not hard to slice it open and bypass the bad part.

Matthew Currie said:

It's normal to have the miss seem to disappear at higher rpm's. It is still probably a non-firing injector.

You can do a very quick check to determine whether the problem is in the injector or the harness, by running the engine, and unplugging the injector you suspect is bad. If it's a bad one, nothing will change. If it's a good one, the engine will briefly bog, then recover. Now swap the connector that tested bad with its nearest neighbor, start the engine again, and repeat the test. If the same plug is dead, it's the harness. If the same injector is dead, it's the injector.

Click to expand...

Yeah, that's what the FSM said to do as well. The problem is, as I unplugged the injectors one by one, each time there was a small drop and then recovery. I did it on all 6 and the nothing changed between all 6. After firing it up again, the problem was gone. Again, this morning I went to get coffee. The jeep drove great on the way there, but by the time I got home I had the CEL and rough idle again. =(

Matthew Currie said:

Check voltage at the injector plug, and resistance of both positive and negative lines. This system switches the negative side, and positive is constant-on with ignition, supplied via a line with branching splices to each injector plug. The splices can go bad and give you low voltage at the injector. You should check resistance of the pos. line back to the connector at the coil, and the resistance of the neg. line back to the PCU.

Injector 1 goes to pin 16 on the PCU. #2 goes to pin 15, 3 to 14, 4 to 13, 5 to 38 and 6 to 58. The positive line to all injectors is dark green/orange, and traces back to the ASD relay, as well as one of the pins on the coil/distributor plug. With ignition on, engine off, you should get a full 12 volts between the hot terminal on the injector plug and ground. If it's a bad splice, it might start out relatively good when cold, and deteriorate as it heats up. If you have a bad wire or splice in the harness, it's not hard to slice it open and bypass the bad part.

Click to expand...

Gonna try that this afternoon. I'll let ya know what happens.

SanDiegoXJ said:

Yeah, that's what the FSM said to do as well. The problem is, as I unplugged the injectors one by one, each time there was a small drop and then recovery. I did it on all 6 and the nothing changed between all 6. After firing it up again, the problem was gone. Again, this morning I went to get coffee. The jeep drove great on the way there, but by the time I got home I had the CEL and rough idle again. =(



Gonna try that this afternoon. I'll let ya know what happens.

Click to expand...

My stepson's 93 had the same problem which turned out to be a bad splice in the harness. I won't go through all the things I did to try to fix and diagnose it, except to say that after I was done nothing on that engine needed tuning up or replacing for years to come, and I still have a spare junkyard ECU! It turned out to be a bad splice in one of the positive lines to an injector, which had just enough resistance in it to allow it to work when cold, but it heated up as it went, and the resistance would rise just enough to cut it out. It would often run just fine for a half hour or more, and then appear after it had been parked and shut off, when the engine bay heats up. I even tested it with an injector tester and it passed, but the voltage when it cut out was something like 9 volts, just enough to cause the misfire. I finally caught it with the wire swapping test when it was hot, and then took an ohmmeter reading to confirm. After many days and many bucks of farting around, the actual repair - a soldered-in bypass for the bad splice - took about 10 minutes.

Good luck on this.

Hey there,

Did you get your problem fixed? I have a 1994 with 132k and am having the same problem.

thanks.

Good Morning BUMP.

scoon07 said:

Good Morning BUMP.

Click to expand...

PM(private message) him

are you getting a code 27 or just a ramdom missfire?

have you checked/tested the fuel injector wiring harness?

have you checked/tested anything?

Hey there,

I have checked and tested everything but the wiring harness. I have put in new plugs, wires, dist cap and fuel filter. Done a fuel pressure test. Put in a new TPS and eCTS. I have checked each injector by unplugging them one at a time. All of them work.

I think my next step is to check the wiring harness, unfortunately, this is my DD and I have very little time to work on it. I am trying to avoid going to the mechanic and just wanted to see if others code 27 got fixed. BTW, I always get a code 27 error.

Thanks for the input.

bump!! having this problem

Hey there,

Unfortunately I sold the Jeep to another Jeep guy who had more time but he did confirm there was a short.

Hope that helps,

Skip

I drive a 92 Jeep Cherokee and I just wanted to give you a big thank you! I am a 19 year old girl college student who was able to fix this problem all by herself thanks to you! Turns out one of my injectors was plugged in upside down and wasn't actually plugged in...but I wouldn't of found that out without you! No more engine light and smooth riding from now on woot woot! It's amazing how much of a difference it made.