Problem after tranny swap - contest

[old_man]

As most of you know, I have been working on vehicles for 45+ years so I though I would take this problem and pose it to you guys. I have a pretty good idea what it is and will troubleshoot it when I get off work today, but thought it might be fun to see how you do. The winner gets a 6 pack of Fat Tire.

I just finished replacing a finiky acting AW4 in my 85 XJ w/4.7L stroker with headers and 2.5" exhaust all the way back. It was running fine before the tranny swap. I disconnected the battery before starting working on it. When I start it, it idles fine and the rpms adjust to 700 rpm just like it supposed to. The problem is that it simply won't rev over about 1500 rpms. When it hits around 1300 it starts to miss.

Fuel pressure seems fine but will put a gauge on it later. Checked TPS connections but didn't measure yet. Nothing replaced other than the tranny.

Lets see your ideas. Propose one fault only and be specific. Follow up your selection with how you arrived at your answer.

 

[JJacobs]

Sounds like a slow leak in the right rear tire.

 

[Apostle]

messed something up taking out.putting in trans. grounds maybe, or a vacuum line. dont see how changing the trans by itself could cause this, but then again, i didnt think a downstream o2 sensor could cause all the problems i had

 

[KingA]

Did you play with the CPS any while doing the swap?

Maybe the crank sensor alignment is off a bit. Sees the flywheel teeth fine at slower speeds but starts to get confused when it spins faster? Loose connector on it or weak signal?

When I was in that area once, I managed to nudge the connector cable too close to the manifold. It melted the cable enough to give me intermittent stumbling at higher revs.

Good luck!

 

[YELLAHEEP]

AW4 in my 85 XJ w/4.7L stroker.

Owner needs to step away from the whiskey bottle......... that combination never existed.........

 

[mj88719]

x2 on the crank position sensor.

 

[old_man]

Owner needs to step away from the whiskey bottle......... that combination never existed.........

Tequilla bottle

 

[old_man]

Hint.....vacuum at an idle is 14". When you rev it up a bit and slam the throttle closed, it bumps to 15.5" just for a second.

 

[YELLAHEEP]

Mass air pressure sensor?

 

[JJacobs]

Mass air pressure sensor?

:confused1

manifold air pressure sensor. GM guys, I swear..

Tom and I were just hashing this one out and it's almost certainly the Kuhneutson regulator latch valve.

 

[dennis461]

transfer case vacuum lines.

 

[RNMedic]

Fuel line pinched/kinked with trans removal and replacement

 

[JJacobs]

The winner gets a 6 pack of Fat Tire.

If one of you out of state guys gets it Tom might want a few bucks for shipping..

 

[old_man]

I got it fixed. I thought I knew what happened but as it turns out, I was wrong. Since I had to drop the exhaust and ended up cutting it and welding it to tweak the fit a bit, I figured the internals had collapsed and plugged the CAT. The low vacuum had me fooled. To troubleshoot, I pulled the O2 sensor and saw no change. Not believing I could be wrong, I unbolted the downpipe and tried again with the same results.

Before I did any of that, I used my SnapOn Renix scanner and verified all the sensors as being within limits.

Being an electrical engineer, I deal with Hall Effect sensors all the time. They normally work at higher speeds and crap out a low speeds, so I figured since it idled fine, the CPS had to be fine. The item that tends to work at low rpms and crap out at high rpms is the coil. I didn't have a spare coil but I had a spare CPS, so I changed the CPS, knowing that wouldn't fix it. I was wrong.

It was the CPS. It wasn't abused or anything and was only about 6 months old. I took it out before removing the tranny and put it in after putting in the new one. It was just a wierd coincidence that it started acting up when I changed the tranny and acting up in a wierd way.

I guess KingA wins the 6 pack. Are you going to be at ColoradoFest to collect?

 

[JJacobs]

Heh- wasn't that my first idea on the phone? I'll take the sixer, then you don't have to ship it.

 

[old_man]

As it turns out, it isn't a Hall Effect sensor as I thought. It is a variable reluctance sensor. Sorry for the bad info.

I found aftermarket suppliers for the high altitude sensor, but nobody has it in stock and I need it to get here by Friday. Bummer. I guess I'll just have to run the old junkyard pull off sensor for ColoradoFest.

Jon, just for you, I'll have a sixer for you at ColoradoFest also.

 

[KingA]

I got it fixed. I thought I knew what happened but as it turns out, I was wrong. Since I had to drop the exhaust and ended up cutting it and welding it to tweak the fit a bit, I figured the internals had collapsed and plugged the CAT. The low vacuum had me fooled. To troubleshoot, I pulled the O2 sensor and saw no change. Not believing I could be wrong, I unbolted the downpipe and tried again with the same results.

Before I did any of that, I used my SnapOn Renix scanner and verified all the sensors as being within limits.

Being an electrical engineer, I deal with Hall Effect sensors all the time. They normally work at higher speeds and crap out a low speeds, so I figured since it idled fine, the CPS had to be fine. The item that tends to work at low rpms and crap out at high rpms is the coil. I didn't have a spare coil but I had a spare CPS, so I changed the CPS, knowing that wouldn't fix it. I was wrong.

It was the CPS. It wasn't abused or anything and was only about 6 months old. I took it out before removing the tranny and put it in after putting in the new one. It was just a wierd coincidence that it started acting up when I changed the tranny and acting up in a wierd way.

I guess KingA wins the 6 pack. Are you going to be at ColoradoFest to collect?

Hehe, sounds like I just got lucky. Glad you got it working. I hate those crank sensors with a passion. Back in the day, Fat Tire was my most favoritest in the world! But alas, no more drinky for me. Spread the love at ColoradoFest for me though! I'm not going to be able to make it.

Adam