HunterRivett

1998 XJ, 4.0 I6, 4x4, Auto, Sport

Purchased (Would run, but terribly. Original engine overheated leaving average cylinder pressure at about 40psi. Would start up and run fine though.)

Replaced:
Motor (1996 XJ Motor Put In)
Spark Plugs/Wires
Cap/Rotor
Oil Change
Coolant Flush

Then would hardly start, and when it did, would hardly run. Upon acceleration, very rapid backfiring occurred at 1500 rpm and it died.

Then Replaced:
Catalytic Converter
Downstream o2 Sensor (have not replaced upstream yet)
Pick-Up Coil
Crankshaft Position Sensor

Would start better, however ran the same.

Then proceeded too:
Fill her up with Lucas Gas Treatment and Seafoam
Remove intake and respective sensors, clean everything, and boiled the intake.

Starts and runs fine, however, she dies if the idle is under 900 rpm, I have it manually set at 1000 rpm. Idles kinda rough. When driving, does fine until the torque converter locks up in 3rd, and will almost never shift to 4th. Stalling and dying happens mostly in the higher gears, with erratic shifts at high rpm being most common. Bucks, shakes, and dies when under load or needing to shift. Brake lights do not work, right low beam does not work, and left high beam does not work (wtf.) Speedometer doesnt work, however, when I apply the brake, it jumps to what it should be (very wtf.) When gas is applied in park, it takes a long time for her to come back down from her revving. No codes. I am truly bamboozled.

I would love it to be a bad brake sensor, dirty connectors on the cluster, and an overdue tranny service, but I have my doubts.

PCM? Bad grounds? Battery? Or a single wire that has been clipped, frayed, burned, or crossed in the jungle of wiring?

Thanks (a lot) in advance.

4OHI6

I would verify the timing is set correctly and also check plugs as backfiring could have ruined one or more. Also verify correct plugs and gap is set.

Did you hear this engine run before putting it in yours?

4OHI6

Also check the IAC. Clean first, but may need replaced.

chipmaker

Which ECM are you using? Is the 96 motor an H.O.?

Bobolink

Anything after 1990 is HO. How did you "set the idle at 1000RPM"? My guess is that you used the little throttle plate stop screw on the throttle body. Therein lies your problem (or part of it). Your idle problem is more likely a maladjusted throttle position sensor, and that cannot be overcome by opening the throttle plate. The stop screw is there to keep the throttle plate from closing so far that it will bind in the throttle body throat. That is its only legitimate function. When it is opened in an attempt to raise the idle speed, it overrides the IAC's function, and the ECM cannot control the idle speed based on engine load (A/C or automatic tranny load) or engine temp, etc. With the '96 or '98, you should have the OBD I or II interface for diagnostics. I'm not familiar with that, since I have a '90 Renix. But I do know that the TPS works the same on both. My TPS would be adjusted with a digital volt meter. Hopefully someone with more know-how with the Chrysler HO system will step up.

oldguy52

96 is the first year to be full ODB II so either the 96 or 98 ECM I would think should work. With kind of back firing I would double check firing order on your plug wires as well as your fuel injector connections. Did you label the fuel injector connectors before disassembly?

HunterRivett

Plugs are a little white, too hot? Copper Champions gapped at .35. Heard the engine, it ran fine. It has checked out fine physically. Timing is set perfect. When the timing mark (yellow) is checked with a gun, it steadily advances to the left of the viewer until about 2500 rpms, then becomes sporadic and crazy. So i assume it is not advancing correctly. I have cleaned the IAC. Multimeter tested fine. It is a 98 XJ with a 98 ECM. The idle has been adjusted with the stop screw at 1000rpm. If taken down to its factory 800rpm, it sputters and runs for a minute then dies. Backfiring is more or less gone.

HunterRivett

The speedo, brake lights, and headlight problem make me believe it isnt as mechanical as we may think. It makes me think the problem is electrical. Id rather buy an ECM than 5 sensors.

HunterRivett

96 motor in a 98 XJ. However, all these new parts I purchased are per a 98. Could that be the problem? A 98 CPS (and pick-up coil, IAC, etc.) on a 96 motor?

Radi

Got a vacuum leak somewhere? Also definitely disassemble, clean and reinstall all grounding points, sounds like some issues there too.

phletch

Replace the upstream o2 sensor.

Larry's XJ

This ^^^ lot cheaper than shotgun aproach on ecm or sensors

Bustedback

The 40psi compression average isn't a red flag to anyone? If you have a dead hole no amount of sensors and potions will make the engine run good again.

CCKen

Indeed.

The OP needs to explain what is meant by 40 psi average (?)

Edit: Explain how the compression check was performed and what was the compression numbers for each cylinder.

HunterRivett

40 psi was on old motor. Now pressure is 120-150. Ive gone through and checked all grounds, hoses, and joints I can. Does anyone have a good diagram of vac system and grounds that Im sure ive missed?