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97 jeep 4.0 auto engine swap - runaway revs
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deep_weeds
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Jun 29, 2012, 3:47 PM
Post #1 of 13
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97 jeep 4.0 auto engine swap - runaway revs
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Hi, folks - brand new here. Looks like a pretty amazing site.
I've just done an engine swap on my 97 Grand Cherokee 4L auto. Old motor blew at just shy of 500k km. New motor bought locally from A Guy; he says it's from a 2000 GC with a standard tranny (thus no flex plate) and has 139k on it; 2000 doesn't fit with the location of the AC and stuff up front according to Haynes (fwiw), but I'm not hung up on that. I kept my old harness, so I had to swap over a bunch of stuff from the old motor - distributor pickup, coil, (oil?) pressure sensor next to the distributor - because all the connectors are different. I also swapped over my old intake manifold for the same reason, though the big plenum under the throttle body on the newer one probably means better performance.
To the problem:
I just started it up, and the revs immediately ran away on me, up to 4000-5000rpm, at which point it was doing some nasty backfiring. Did the same thing on repeated tries, and I shut it down immediately each time. I could use some leads on what might be causing it. (For the test I didn't bother with the belt or rad in case I have to move the motor again.)
Two things I'm concerned about: The first is that I think, but I'm not completely certain, that I got the position of the flex plate on the crank right. After putting it together I grasped its importance in feeding timing & position information to the computer. If I got it in one of the five wrong positions, could that account for the runaway? Neither Haynes or the factory service manual appear to describe the correct orientation of the flex plate on the crank - Haynes just says "mark it so you get it right when you put it back together", which obviously doesn't help in this case.
The other is that (due to circumstance) I didn't put on a new manifold gasket - the old one appeared to be in reasonably good shape - when I did the intake transplant. I didn't swap the exhaust manifold - the one that came with the new motor has the bellows in it and I stuck with it. I did torque the bolts to spec (24ftlb), but I realize that if the if the height of the points on the exhaust manifold where the bolts clamp it don't match those of the intake, then torquing to spec may be meaningless, and there could be leakage in the intake.
Incidentally, it doesn't seem to be throwing any codes.
I'm really trying to put this one to bed. Can anyone give me a lead?
Thanks very much.
Jonathan
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Hammer Time
Ultimate Carjunky
/ Moderator
Jun 29, 2012, 4:06 PM
Post #2 of 13
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Re: 97 jeep 4.0 auto engine swap - runaway revs
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You have either a stuck throttle plate, damaged IAC, severe air leak or incompatible operating system.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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deep_weeds
Novice
Jun 29, 2012, 5:12 PM
Post #3 of 13
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Re: 97 jeep 4.0 auto engine swap - runaway revs
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You have either a stuck throttle plate, damaged IAC, severe air leak or incompatible operating system.
I'm on my own, so I can't look at the throttle plate when it runs - I'll have some help tonight and can confirm. Feels fine, though - the cable moves it freely and it springs back hard.
Ignition computer: When the motor blew we knew it was high-miles and about time. No reason to think there's anything wrong with the electronics, though. I'm not a professional mechanic, but I am a professional embedded systems engineer, so I have fairly good feel for when something's on the fritz, and this one appears to be okay. The startup sequence looks fine, it goes through the error-code reporting sequence... nothing untoward. But I do acknowledge that outward appearance isn't definitive.
Severe air leak: Could be the case, given my reuse of an old gasket and the possible incompatibility between my intake and exhaust manifolds. Would taking a vacuum gauge to it be helpful?
Incompatible operating system: I don't know whether to take that as a joke, since what I swapped into this vehicle is the block (with distributor) and exhaust manifold. Not much there to be incompatible with.
Are you dismissing the possibility of my mislocating the flex plate on the crank? Obviously I'll be delighted if I got that right, because I won't have to pull the motor again.
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deep_weeds
Novice
Jun 29, 2012, 7:05 PM
Post #5 of 13
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Re: 97 jeep 4.0 auto engine swap - runaway revs
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Are you dismissing the possibility of my mislocating the flex plate on the crank? Yes, I am. That couldn't cause a symptom like that.
Well, that's excellent news. I didn't want to pull the motor again, though I've since read that the crank/flywheel bolt pattern isn't regular, so it's supposedly not possible to misorient it.
Don't blow off the incompatibility idea so quickly. There are numerous ways for things to be incompatible. Those blocks can be different and engineered for different system. It's probably not the case here but I have seen crankshafts with different castings for mating to different sensors and computer systems that can't be retrofit into older systems.
Okay, fair enough. Just sounded unlikely in this case, given that this iron-six is about the most bog-standard motor around and strikes me as probably immune to the sort of subtlety you describe.
Try unplugging the idle control to determine if the computer is doing it intentionally or not. Look carefully for any air leaks.
I don't know where the idle control is or what it looks like. Can you direct me to it?
Is the computer setting any codes at all?
I don't have a proper reader, so I'm just going by the on-off-on-off-on ignition key sequence that reports (a subset of the codes) via the odometer, and it's reporting nothing.
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Hammer Time
Ultimate Carjunky
/ Moderator
Jun 29, 2012, 7:08 PM
Post #6 of 13
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Re: 97 jeep 4.0 auto engine swap - runaway revs
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There can be many differences in the head castings too.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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deep_weeds
Novice
Jun 29, 2012, 9:27 PM
Post #7 of 13
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Re: 97 jeep 4.0 auto engine swap - runaway revs
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Alright, so based on your suggestions, I'll get a new gasket under my old matched pair of manifolds tomorrow morning and let y'all know how it goes.
Sure is a PITA doing these with the engine installed. Would have been so much nicer to do it before it went in...
thx.
jl
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deep_weeds
Novice
Jun 29, 2012, 9:30 PM
Post #8 of 13
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Re: 97 jeep 4.0 auto engine swap - runaway revs
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Oh - I was having network problems earlier tonight and couldn't see the graphic re: the idle air control. I get it - that just bypasses the throttle plate. I'll take a good look at it tomorrow as well.
thx.mch.
jl
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deep_weeds
Novice
Jun 30, 2012, 8:37 AM
Post #10 of 13
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Re: 97 jeep 4.0 auto engine swap - runaway revs
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I understand what you're saying. Given the circumstances and the pieces I've been juggling, I'd feel a lot more confident moving forward first with the right combination of manifolds and a fresh gasket. Then if that doesn't solve it, I'll at least be certain that leakage isn't the problem and I can look at the idle air control.
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Hammer Time
Ultimate Carjunky
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Jun 30, 2012, 9:41 AM
Post #11 of 13
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Re: 97 jeep 4.0 auto engine swap - runaway revs
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No, I'm saying nothing of the kind. I'm merely pointing out things that needs to be looked into. You're not at a stage to start changing anything until you understand more about what is or isn't happening.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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deep_weeds
Novice
Jun 30, 2012, 12:25 PM
Post #12 of 13
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Re: 97 jeep 4.0 auto engine swap - runaway revs
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Understood. That turned out to be the solution, though. I got a fresh gasket and installed the previously-matched pair of manifolds, and it's settled down quite nicely.
Thanks very much for the guidance.
Jonathan
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Hammer Time
Ultimate Carjunky
/ Moderator
Jun 30, 2012, 12:36 PM
Post #13 of 13
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Re: 97 jeep 4.0 auto engine swap - runaway revs
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Glad it worked out.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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