New Engine Won't Run Right

I've just put a rebuilt engine into my 1990 XJ w/4.0 automatic. It starts, but runs really rough. If you try to increase the throttle it runs slightly weaker without any increase in rpm's. I talked to the place I purchased the engine from today and they immediately targeted the Crankshaft Position Sensor.

That got me thinking about the "flywheel" and all the holes in it that the CPS uses. I checked my factory service manual so see if it said anything about the alignment of the flywheel and it was mum on the subject. So I'm asking... Is the alignment (in relationship to the crank) an issue?

We've double checked the distributor alignment and the injectors are connected correctly. We've checked compression and it's good at 140psi in each cylinder. We checked the plugs (new) and they are all fouled with black soot. All the vacuum hoses have been checked and double checked.

Am I missing something else?

Answering a bit of my own question after googling and reflecting on the build. The flexplate only attached to the crankshaft in one position as the holes are indexed. This leave me just the CPS as suspect.

Or the fuel injectors.

My friend's 97 XJ....total motor failure, didn't blow anything through the block but somehow the #6 piston was GONE....all that was left was the rod!

Anyway he picked up a motor from anothre 97 and swapped it in. Low mileage, etc. Was running rough and terrible, throwing misfire codes for each cylinder. Swapped fuel rails back to the origional motor's fuel rail and injector and all was well.

So something about injectors sitting around too long after being used can make them work improperly?

That OR the CPS is likely to be at fault I'd assume.

You need to start the testing process. I would start with a fuel pressure test. Easy to do on the schrader valve on the fuel rail. Test with engine both running and not running; compare to factory specs. and I'd also test and adjust your throttle position sensor. The TPS is adjustable on yours; has that been done yet since you fired it up??

I would suspect something else besides the crankshaft position sensor. Not saying it isn't that but "normally" if it's the crank sensor, the engine will either die or misfire when losing the crankshaft reference signal. The crank sensor can be tested easily for resistance if you are so inclined.

Crankshaft Position Sensor: Common Symptoms if bad:
Both the fuel gauge and or voltage gauge may not work/display

It is possible that you may see a No Bus on the odometer

You will have no spark. Fuel pressure may check okay but fuel won’t get to injectors

Sometimes the OBDII code reader has trouble connecting to /reading codes
Crank sensors can be intermittent; "thermal failure" is pretty common. Means that the sensor fails when engine gets hot, but works again when cooled back down.


CPS TESTING PROCEDURE 1991 – 2001 4.0L H.O. engines

1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C (A-B-C) looking into connector left to right with the part with the notch in the middle on the right) Ohmmeter should be set to 1K-to-1OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

CPS TESTING PROCECURE for 1987 – 1990 4.0 L engines

Test # 1

Get a volt/ohm meter and set it to read 0 - 500 ohms. Unplug the CPS and measure across the CPS connector's A & B leads. Your meter should show a CPS resistance of between 125 - 275 ohms. If the CPS is out of that range by much, replace it.


Test # 2

You'll need a helper for this one. Set the volt/ohm meter to read 0 - 5 AC volts or the closest AC Volts scale your meter has to this range. Measure across the CPS leads for voltage generated as your helper cranks the engine. (The engine can't fire up without the CPS connected but watch for moving parts just the same!) The meter should show .5 - .8 VAC when cranking. (That's between 1/2 and 1 volt AC.) If it's below .5vac, replace it.

reset ur computer and make sure it will work w/ the new engine!