87 jeep cherokee starting problems after swap

[speeddemontbn7]

ok, this will be fun to tell you all. I just finished swapping in another 4.0 from another jeep. I did a .040'' bore and put in forged pistons and rods, new cam, lifters etc. bolted the thing back in the car, connected everything, no go! What it does is very odd. It will start in a normal amount of time and run for literaly one half to one second, then die. It is almost as if if fires once on each cylnder then dies. I am getting an accurate reading from the cps and the timing chain i know is aligned (checked 3 times). I am doing this project at school and my teacher ditched for the semester. Someone please help!?!? If any more information is needed let me know.

[Saudade]

So they teach you overhauling but not troubleshooting?

Sounds like you're not getting fuel. Make sure all your lines are connected properly. Check the fuel pressure with the key on (assuming it MPFI) at the injector rail. There should be about 31psi and your injector harnesses are connected in the correct order.

Try spraying some starter fluid down the TB while cranking and see if it still runs as long as you spray.

Also, double check all of you sensor hookups including their harnesses. You might have pinch/chaffed one.

Distributor alignment is critical too. Double check that.

[speeddemontbn7]

Originally Posted by Saudade

So they teach you overhauling but not troubleshooting?

Sounds like you're not getting fuel. Make sure all your lines are connected properly. Check the fuel pressure with the key on (assuming it MPFI) at the injector rail. There should be about 31psi and your injector harnesses are connected in the correct order.

Try spraying some starter fluid down the TB while cranking and see if it still runs as long as you spray.

Also, double check all of you sensor hookups including their harnesses. You might have pinch/chaffed one.

Distributor alignment is critical too. Double check that.

Thank you, i will try those, how critical is distributor alignment?, we rotated the assebly to tdc on #1 the best we could, but it may not be exactly aligned. I figured that it was fuel because it has good compression and i can give it more air by opening the throttle, (does nothing) so i will check distributor and the pressure.

[speeddemontbn7]

Btw, the course is titled 'engine rebuilding' we covered troubleshooting minimally, that comes later down the road in 'advanced diagnostics'. Does anyone know which color wires go to which cylnders injector? i have no knowlege when it comes to electrical diagrams.

[oljeeptek]

Originally Posted by speeddemontbn7

ok, this will be fun to tell you all. I just finished swapping in another 4.0 from another jeep. I did a .040'' bore and put in forged pistons and rods, new cam, lifters etc. bolted the thing back in the car, connected everything, no go! What it does is very odd. It will start in a normal amount of time and run for literaly one half to one second, then die. It is almost as if if fires once on each cylnder then dies. I am getting an accurate reading from the cps and the timing chain i know is aligned (checked 3 times). I am doing this project at school and my teacher ditched for the semester. Someone please help!?!? If any more information is needed let me know.

Pay attention boys and girls. I shool ya a little on early Jeep 4.0's.

Distributor alignment is not very critical. Remove the alignment tangs so it can be adjusted and set the rotor so the trailing edge is .020" PAST the #1 plug terminal of the cap with the engine at #1 TDC.

You can turn it with the engine running and not change anything. The ECU adjusts the timing based on the cps, map and knock sensors mostly. The sensor (Called the sync sensor) is for Sequential injection only. If you think it might be bad, disconnect it and start it up. No sync signal = multi port( bank fired injectors). You don't need it to run.

Ignition timing is controlled by the ecm based on the crank sensor at the rear of the engine. If its good, you will get 1.0 volts AC at normal cranking speed. (Not running). You should check this by backprobing the ppl/wt and wt/bk wires at the rear of the engine.(Plugged in.)

This will cause start and die, or no start if bad enough. New sensors typically came out of the box at .7v AC and needed to be tapped closer to the flywheel to get it up to spec. Sometimes this takes a long screwdriver and LIGHT hammer tap to bend the bracket. Don't exceed 1.0V or it'l hit the flywheel when it turns over.

There is/was a service bulliten on '87-early '88 cherokees and wagonneers for def cps (Crank position sensor) wiring. Basically bypass the underhood bulkhead connector on the above referenced wires to the ecu.
Set the tps to .7v at closed throttle and clean the throttle bore if theres ANY carbon in it The idle speed is set with the torx plug on the engine side of the throttle body. You may need to remove the tin plug first, but most are probably accessible by now.
You'll also have to block the iac port to get base idle.

Have fun and save these fir future use. You may need it!

Oljeeptek

[speeddemontbn7]

I have never heard nore read anything about this 'sync' sensor spoken of earlyer. But i am fairly confident that the injector wires are out of order now that i have gotten ahold of the wireing diagram in the idiot book. If this doesnt fix it i will turn to sensor testing. Any other suggestions would be appreciated.

[oljeeptek]

Originally Posted by speeddemontbn7

I have never heard nore read anything about this 'sync' sensor spoken of earlyer. But i am fairly confident that the injector wires are out of order now that i have gotten ahold of the wireing diagram in the idiot book. If this doesnt fix it i will turn to sensor testing. Any other suggestions would be appreciated.

The sensor in the distributor is the sync sensor. It shouldnt matter if the injector wires are wapped to run it. it just won't have proper power but it will still run.
x
Many more options..

Does it have a security light on the dash?

Does it have spark, or injector pulse while cranking/after it dies? Tape the trigger on a timing light and put it where you can see it. Hold the key until it after it dies.

See if it will run on carb cleaner spray. (Use the flammable kind and obviously be carefull!) Don't start any fires.

[speeddemontbn7]

More info monday, car is under lock and key at school, i doubt that it is the sync sensor seeing that it ran before swapping the motor, (same dist.), everywhere i read says be extra carefull not to f*** up the injector wire order.

[oljeeptek]

Originally Posted by speeddemontbn7

More info monday, car is under lock and key at school, i doubt that it is the sync sensor seeing that it ran before swapping the motor, (same dist.), everywhere i read says be extra carefull not to f*** up the injector wire order.

I agree the sync sensor is probably ok. I was only giving you some oversight on how the system actually works, and a few things to check. The injector wires are not critical to running, only running right. You can play mumbly-peg swapping them around and it will start and drive fine. It will only be down on power and fuel mileage. These Renix systems are fairly bullet proof in their simplicity.

You might try a known good map sensor though. I seem to recall they won't run with the map wiring disconnected. (No default mode) Your map sensor should have a green insert at the wire connection point if its the correct one.

Spend some time testing a few circuits and save major time and $$$ (and hair) by not just throwing parts at it.

Good luck.

[oljeeptek]

Hey,
A couple items to clarify:
Cps voltage output cranking .7-.9v AC

Sync sensor disconnected takes 2 full revolutions of the crankshaft during cranking to go to bypass mode, about 30 seconds. I seem to remember the sync shutting down injection if it's seem as way off from the cps. Thats why I disconnect it and retest. I'll have to look up my old notes for more here.

Vehicle antitheft will also disable injectors almost immediately if equipped.

[speeddemontbn7]

is the map sensor harness plug a green three wire plug locared near the valve cover?

[speeddemontbn7]

When i say near the valve cover, i mean near the valve cover by the firewall. I ask this because i had alot of trouble during reinstall of the motor with that connector, i cracked it but i didnt think that it would have made the difference.

[oljeeptek]

The map sensor is the black sensor mounted to the firewall at the back of the valve cover. It is the main fuel input to the engine computer. It should have 1 hose connected to direct manifold vacuum, and a single 3-wire connector plugged directly into it. The insert inside the sensor side of the connector should be green. You will have to use a mirror or turn it over to see it.

It definitely will not work broken. The good news is that is a very common GM part. The ones with a blue insert are baro sensors, and the red ones are vac sensors. They are not interchangeble so get the right colored one.

GM's will run if the map is disconnectly, but I don't think the Jeep ecu will default around it.

[speeddemontbn7]

Replaced connector, car runs, but ony 10psi oil pressure...

[oljeeptek]

Originally Posted by speeddemontbn7

Replaced connector, car runs, but ony 10psi oil pressure...

Ooohh, Bummer on the oil pressure!

Dd you get 'er fixed?