thepeaceseeker

Hi everyone. Just joined the forum and this is my first post. I've gotten good info from this site before so I thought it's time to join up! I hope you can help.

Well, 95 Jeep Grand Cherokee Laredo with replaced 4.0L from a 94. It has been running perfect for 2 years but died a couple weeks ago. It had the common crank position sensor failure. I had replaced many things when I replaced the engine but not that sensor, and not the cam position sensor in the distributor so I took this opportunity to replace both.

It was a real pain getting at the crank sensor but got it changed. After reading some posts on this site, found the easiest way was with several extensions and a swivel adaptor for my socket.

I marked the position of the distributor rotor and replaced it back to the same position after replacing the cam position sensor inside.

Now it runs extremely rough, misses, can't rev it up very high. Spark plugs show black soot whereas they used to show good combustion. Since it was a couple years since I changed the cap and rotor, I changed them too : no difference. Plugs and wires are recently replaced, good condition. I don't have a spark gap tester but seems like they have good blue spark on all cylinders.

I thought maybe I somehow messed up reinstalling the distributor and the timing was off, so I took it out and tried both one cog ahead and one cog back. One way it wouldn't run at all, and the other way it ran equally as bad. I don't have a timing light to check the timing so I just put it back to where I had it marked.

No engine error codes other than when the sensor failed.

I don't have much money right now to replace a bunch of parts or to buy a timing light. I don't know anyone with a timing light either. I do have a compression tester so I checked that the rotor was at plug wire 1 position while at TDC.

Any suggestions where to go from here? It just seems so strange that it ran perfectly before and now runs so rough. The crank sensor would just make it not run at all. To me it seems like the timing is off, and this is causing the improper combustion fouling the plugs. A compression check showed good compression on all cylinders so I don't think it's because the timing chain slipped a tooth on the gear.

What can I do now that won't cost much money? This seems like it should be so simple and is pissing me off! HELP!!!!

Thanks in advance.

ZINGGG

The first thing that comes to mind is to make absolutely positively sure you have the firing order correct. Gotta eliminate with the most basic possibilities first

thepeaceseeker

Yeah, they are correct. I left all the plug wires connected and just disconnected the coil wire when I removed the distributor. I also checked as the firing order is stamped right on the manifold. I also have the Haynes repair manual.

Any other suggestions?

Radi

Where did the CPS come from? OEM? It wouldn't be the first bad-out-of-the-box aftermarket sensor. And yes, they sometimes will still run with a marginal CPS, although very poorly.

macgyver35

After you've been on the forums awhile, you'll notice a lot of posts from people who know telling you that the ONLY place you should ever source a crankshaft position sensor is at the dealership.

I learned the hard way too. Wasted my time and $93 on one from NAPA that didn't work. Took it out and replaced it with one from the dealership ($117) and all of my problems went away.

thepeaceseeker

Hmmmmm.... I guess that makes sense. If the sensor isn't sensitive enough, at higher rpm's it may not pick up the cogs on the flywheel properly and the engine computer will kill the spark.

I'm in Mexico and bought it from a refaccionaria (parts store). Who knows where the hell they sourced it from. I noticed the bolt hole was elongated for adjustment on this one but the original one was not. I guess I'll first check the positioning of it to make sure it is sitting in deep enough. If that doesn't work I'll have to buy another one from a dealer here. That will cost twice as much as the parts store, plus the money I wasted on the first one.

Can't this sensor be bypassed to trick the computer? I don't think the computer actually senses the rpm from this sensor does it? Is it just an on or off signal the computer wants to see or is it actually sensing rotational speed?

hankthetank

It's a position sensor. 3 breaks at 120 degrees tells the ecu where 3 pairs of pistons are.

Radi

Unfortunately it's one of the few it's dead in the water without. CPS tells the PCM when to fire the plugs and is part of the calculation on when to pulse the injectors...without it the engine won't fire.

It's basically a switch that closes three times for every revolution of the crank. If one of those signals goes missing here and there you'll get a misfire or no-fire. If the switch closes a fraction of a bit late you'll have timing issues.

thepeaceseeker

Hey guys, thanks for the input. Yeah, the sensor I bought for 1/2 the price of the dealer was crap! They wouldn't return it either. So now I'm out the price of that sensor. Sometimes is bites you in the *** when you try to save a few bucks.

Like the posts above said, this is one of the most important sensors for the engine. Don't cheap out on it if it fails for you. Buy a made in USA version, not the cheap China crap.

Alot of posts here say that the sensor is either good or bad : either it runs fine or doesn't run at all. BS! They guys who posted above are right. The sensor can be too WEAK and not pick up the notches on the flywheel consistently, causing the engine to just barely run, misfire, not rev up properly, timing problems : especially at higher RPM's.

GOOD NEWS: I'm a pro at changing this sensor now! LOL! I found the easiest way is with a LONG extension (or in my case a few shorter ones put together) and a swivel where the socket attaches. It's a few minute job that way. For putting the bolt back in just put some electrical tape over the head of the bolt before you stick in in the socket. It will stay there until you get it started in the threads. Oh, and I didn't need to unhook the driveshaft or anything like some guys have posted.

Thanks again guys! Cheers!