surging engine

[desertrandy]

I have a 1992 cherokee with the 4.0 engine and auto transmission. I recently installed a remanufactured engine with all new sensors, distributor, coil, wires etc. I drove the engine around town for 300 miles keeping the speed below 55 mph to break in the engine and everything was perfect. Good oil pressure, water temp, volts, everything. Then I took it on the freeway. After 2 miles the engine starting cutting out. The rpm's would suddenly drop and recover, causing the jeep to buck wildly. I got off on surface streets and no problem. Got back on the freeway and same thing, engine starts cutting out. The check engine light never came on so I ended up taking it to a mechanic. They said I had metal shavings in my new distributor that was interfering with the magnetic pickup. I picked up my jeep and drove it home. No problem, ran great. Went out to drive a couple of hours later, backed out of the driveway and all hell broke loose. Engine starts surging worse than ever, form almost stall speed to 2000 rpm's and everything in between. I pulled back in the drive way and let it run until I got a check engine light. Finally after 10 minutes I got a code 14 and 24 indicating voltage problems with the map sensor and tps sensor. I proceeded to change these with new ones and no difference. It still surges. Back to the mechanic and now they say it is the crank position sensor (which I put in a new one when the engine was out) and the ecm or pcm (i can' remember). Has anybody had an experience with this. I have replaced all the sensors and switches and now this pcm or ecm is going to cost 450.00. Any advise would help. Thank You. randy

 

[grahamtwatson]

Did you keep your old sensors? If so you could try swapping them back in, one at a time...

 

[desertrandy]

Yes, I kept them for spares. Good point, never thought of it. I'll give it a try

 

[serge]

Also, I would check the vacuum line that goes the MAP Sensor. If it is cracked or twisted, that would cause the behaviour you observe.

 

[langer1]

More likely than not it's a bad connection somewhere, check the grounds and all the connecters I doubt the ecu is bad.

 

[desertrandy]

I suspected a vacuum leak and checked all the lines. They are fine. However when reassembling the engine I installed new o-rings on the fuel injectors before putting the fuel rail back on the intake manifold. I bought the new o-rings from napa and they were colored black. The Hayne's manual said they should be brown in color. If the o-rings are the wrong size, could there be a vacuum leak at the intake manifold where they are inserted?

 

[langer1]

I suspected a vacuum leak and checked all the lines. They are fine. However when reassembling the engine I installed new o-rings on the fuel injectors before putting the fuel rail back on the intake manifold. I bought the new o-rings from napa and they were colored black. The Hayne's manual said they should be brown in color. If the o-rings are the wrong size, could there be a vacuum leak at the intake manifold where they are inserted?

No that will not cause a voltage problem you need to start there.

 

[serge]

According to the Renix manual (maybe the Mopar fuel system is similar), the brown O-rings are installed on the intake manifold end of the injector, and the black ones are on the fuel rail end. Applying logic, that must mean that the o-rings are different. That doesn't mean that this is true, but it is a place to start...

I wonder if you can single out o-rings as suspects for vacuum leak by spaying some injector cleaner around the injector and observing engine behaviour. Please, take precautions and verify that this will not damage the injector.

 

[serge]

No that will not cause a voltage problem you need to start there.

A vacuum leak will result in different MAP sensor output -- I've verified that when looking for a solution for my '89 XJ shutting down after starting.

 

[langer1]

A vacuum leak will result in different MAP sensor output -- I've verified that when looking for a solution for my '89 XJ shutting down after starting.

True but OP has two codes both saying voltage to high/low and vacuum would not cause the TPS and the MAP voltage to off.

 

[outlander]

According to the Renix manual (maybe the Mopar fuel system is similar), the brown O-rings are installed on the intake manifold end of the injector, and the black ones are on the fuel rail end. Applying logic, that must mean that the o-rings are different. That doesn't mean that this is true, but it is a place to start...

I wonder if you can single out o-rings as suspects for vacuum leak by spaying some injector cleaner around the injector and observing engine behaviour. Please, take precautions and verify that this will not damage the injector.

A less messy approach is to use a unlit propane torch around connections to see if it causes a change in rpm.....if there is a leak the propane will be drawn into the engine which will cause a rpm change letting you know you've found the leak.
Although,
I tend to think you would smell a strong gas odor under the hood if the injector o-rings are leaky.Possibly it wouldn't smell as bad if they are the manifold o-rings that are leaking....I know for a fact that if the upper o-rings(at the fuel rail) are leaking it'll smell like a gas station on wheels(been there,done that,don't wanna do it again)