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ok 1991 jeep wrangler 4.0 6 cyl. problem    Jeep runs great (105.000) miles when I have sea foam in the tank,have been using premium without ethanol but every now and then it runs rough till i put the sea foam in it check engine light said #1 cyl misfire so new plugs coil rail,new injector # 1 cyl then it ran fine for about 1 month then same problem . what I cant figure out is why does it run good when I put sea foam in the tank ?  I dont think it is suppose to run on seafoam alone thanks for any help, even the dealership is baffled

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Hi, Jerry...Seafoam might temporarily resolve issues like a fuel pump sock filter in the tank being clogged or the need to replace an inline filter.  Seafoam could purge crud or make the fuel pick up easier.  (Fuel volume or pressure at the rail falls off with a clogged filter or even a pressure regulator problem.)  An O2 sensor may clean up with Seafoam running through with the fuel mixture.  

The #1 misfire has been a chronic problem with the Jeep MPI 4.0L and not always easy to diagnose.  There is a lengthy discussion at our YJ/TJ Wrangler forum that casts considerable light here.  This error code is often erroneous.  Check out the topic and many solutions discussed.  We can go from there:

 

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ok thanks moses also this engine has no distributor cap what is this called I have never seen this before as for the original problem it act like it is starving for fuel to me I thought of replacing the fuel pump do you think it would be a good Idea? thanks Jerry

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Jerry...Is your Jeep Wrangler a 1991 4.0L inline six?  The original engine would have a distributor.  Through 1999-2000, depending upon which Jeep model, the 4.0L inline sixes have a distributor with a cap and plug wires leading to each spark plug.  From 1999/2000-up, Jeep engines use a "distributor-less" or "coil-on-plug" (COP) ignition.

Unless your Jeep has a later transplant COP engine, you should have a distributor.  It will be on the right/passenger side (LHD models) of the engine, midway between the block ends.  If instead you have what looks like a "stub" shaft and housing rising from the block in that position, this is the camshaft position sensor, which also serves as the drive for the oil pump.

Post some photos if you need further clarification...Glad to help identify the equipment.

Moses 

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Hi moses call me stupid my jeep is a 2001 sorry it runs good when I put the sea foam in the tank but if I dont it dies at a stop light . I already replaced the coil rail plugs # cyl injector it is the original engine I thought I would replace the tps sensor, the co2 sensors and see what that does if that dont fix it maybe the fuel pump? thanks Jerry

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Hi, Jerry...Are you still getting MIL/Engine Check codes?  One of the common problems with the C.O.P. 4.0L engines is the camshaft position sensor.  This is the device that replaced the distributor.  The 4.0L engines need to drive the oil pump from the distributor, and eliminating the distributor required an alternative.  Since camshaft position is also a late emissions concern and part of the sensor system, the camshaft position sensor doubles as both the oil pump drive and a practical point for measuring the camshaft position relative to the #1 piston and crankshaft position.

Camshaft position is compared with the crankshaft position (CPS reading).  If there is too much wear on the timing chain and sprockets, the camshaft will be retarded timing.  This is picked up by the camshaft position sensor and will result in a MIL code indicating a defect here.

The cam position sensor is a wear item and has been known to cause trouble.  You're not getting that code, however.  If it comes up, you'll know what it is.

In the YJ/TJ Wranger forum (please post your TJ Wrangler topics there for better member support), there is considerable discussion about the #1 Misfire Code.  I provided a link to that topic in a post reply above.  Review the #1 Misfire Code remedies. 

Since the Seafoam seems to restore performance, the O2 sensor is more likely an issue.  The TPS is a failure prone component.  Also make sure the exhaust system is not restricted. 

You're on the right track...

Moses

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