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4.2L Jeep Engine with Mopar MPI Conversion Stalls When Coming to a Stop


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I am writing about my 1984 CJ7, 4.2 I6, manual trans with the Mopar fuel injection kit installed. The Jeep has been in storage for the last nine years and I am getting her going again. I had to do some engine work on it, but this is not the source of my problem. Since getting it running again (she runs well sitting still and accelerating under load) A peculiar problem has emerged. When I slow to come to a stop the engine dies and it is only when coming to a stop.

Since it sat without running for so long, I have removed the fuel tank and cleaned, replace the sending unit and all hoses, installed a new filter, a new fuel pump, a new pressure regulator and an idle control air valve. The engine always restarts immediately. This problem didn't exist before having to put it into storage.

I am at a loss as to what else to look into. Any ideas?

Thanks

Terry Utecht      Littleton, CO

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Terry...I would check the vacuum hoses and the MAP sensor voltage.  You changed the idle air control, but I would also check the throttle position sensor (TPS) voltage settings.  The TPS is a known wear item.  Setting long periods can cause stickiness or corrosion.  Test these two devices before replacing them. 

Check connectors, especially any butt connectors.  I use rosin core solder and double up on heat shrink tubing for any EFI connections.  Butt connectors create resistance when they are new.  Setting for long periods can create corrosion and higher resistance in critical EFI wiring.  

Also check the pickup on the crankshaft position sensor (at the crank damper).  Corrosion or a poor gap can create havoc.

Let us know if this turns up a problem.  We can go from there...

Moses

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  • Moses Ludel changed the title to 4.2L Jeep Engine with Mopar MPI Conversion Stalls When Coming to a Stop

Moses,

Thanks for the speedy reply. I have spent days trying to figure this out and now have a few more to think about. I have spoke to Hesco about this and they gave me a few things to check/replace as well. One of these is the transfer case mounted speed sensor. Thoughts? I will check the vacuum hoses as I don't have many this shouldn't take to long to accomplish. 

I have noticed that this problem seems worse when the engine and outside temp are warm to hot. ( It has finally started to warm up in Colorado) Can you give me what the voltages are supposed to be for the TPS & MAP sensors? The reason this was stored in the first place was due to the #5 piston fragmenting. All six pistons were replaced with the block in the Jeep. I left the crank, camshaft, timing chain and distributor in place so as to avoid any timing issues. The head was reworked and reinstalled along with the original MPI intake. I did remove the crank sensor, cleaned and reinstalled at the distance Hesco gave me of .025. According to them, this is a either it works or it doesn't sensor. I don't have any butt connectors I can remember. The kit had all of the necessary connectors on the supplied wiring harness.

Most of the parts for this kit can be found at your local parts store. You just have to know what year Jeep the parts are.

Terry

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Terry...The two-rail EFI system with a pressure regulator (return fuel from the rail to the tank) is a 1995 YJ Wrangler prototype with a 60-way computer.  TPS and MAP voltages would be like a 1995 YJ Wrangler.  The later single rail system is essentially a 1997 TJ Wrangler prototype, although it still uses the 60-way PCM.  Part numbers on the MAP and TPS should help identify the part and year application. 

 A FSM will show voltages shown in troubleshooting sections for the MPI system.  HESCO may have these figures as well.  I highly recommend that once you identify the prototype EFI system, you find a CD FSM online (typically eBay) for your EFI model prototype.  FSM CDs are inexpensive, easy to navigate and deliver factory information.   Here are a few examples currently at eBay.  The Korean version is really cheap, may be worth seeing how complete it is:

https://www.ebay.com/itm/233988867418  [Korean, presumably an FSM copy?  For $9.90 a bargain if actually Mopar content and if navigable.  This is a compilation of 1991-95, sometimes questionable but definitely the YJ Wrangler cohort similar to 1995.]

https://www.ebay.com/itm/130741196171  [Bishko is clearly a PDF factory manual, easy to navigate in CD or download version.]

I would at least check the transfer case mounted speed sensor.  HESCO is right that this could influence stalling when the vehicle comes to a stop.  Tire size and axle ratio would play a role here.  Did you change gearing or tire size dramatically after the storage period?  Check the VSS wiring and connections for corrosion, nicks, burns and such.  Do the same with wires that were disturbed during the cylinder head removal. 

CHECK ALL GROUNDS:  Battery to engine, engine to body and engine/body to frame.  Grounds are as important as positive leads on a D.C. system.  Loose or high resistance grounds can interrupt circuits and throw off PCM data readings.  Fiberglass and plastic parts create non-conductive havoc.

Moses

 

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  • 1 month later...

It has been a month since I first posted on this subject. 

I have replaced the TPS and checked and re-checked all grounds and the problem still existed. 

After one more call to Bennie at Hesco, in my opinion this man is a genius, he instructed me to drill a 3/32 hole in the butterfly plate. I live at almost 6,000 feet and his feeling on this was that the engine was being starved for air. He was right, drilled the hole and the problem went away immediately. 

I would like to ask you one more question on this engine being hard to restart when hot. In my first post I mentioned all of the fuel components I had replaced. It will restart but you do have to crank it for quite some time. The engine doesn't stumble when it restarts and idles as expected. I have had the Jeep emission checked and it passed with flying colors, but the technician did say that he didn't think it was going to restart at one point, he had killed the engine during the test. Manual transmission difficulties on his part!

Any ideas?

Thanks

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After letting it sit for about an hour, I did a pressure check at the rail. The pressure was 0. Turned key to on, I could hear the pump cycle and the rail pressure was still zero. I did this a couple more times, key to on, pump cycled and rail pressure 0.

I then cranked the motor and the pressure slowly built to 52 psi and the engine started. Ran perfectly. Turned key off and the rail pressure dropped to 10 psi in short order. Turned key back to on and the rail pressure went back to 25 psi. Engine started and ran fine.

As I stated earlier, the fuel pump and pressure regulator are new. Could this be a check valve issue in the pump? Is the regulator, or pump,  allowing the fuel line to drain after running? Would you recommend an inline check valve between the pump and the regulator?

Thanks again.

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Tdu659...Very pleased that Bennie helped solve your problem.  Interesting that the MAP, IAC and barometric feedback from the PCM does not compensate, but maybe those functions are curtailed or limited in the late conversion kit.  I recall that the late kit still uses the 1991-95 60-way PCM with the later (OBD-II era, 1997 up) single rail EFI...If the hole solves the problem, Bennie has likely encountered this before on high altitude applications. 

Regarding the pressure drop, which EFI kit do you have?  The later single rail type with the regulator/filter unit remotely mounted near the fuel tank?  This regulator is essentially a 1997-up TJ Wrangler type tank/module regulator with the EFI conversion kit's machined adapter housing, which allows remote mounting of the filter/regulator...The earlier two-rail systems have the regulator at the fuel rail and a return fuel line to the fuel tank.

The factory in-tank pumps do have a check valve that helps prevent fuel and pressure drainback.  Remote "universal" pumps will often have a check valve as well.  (You mention changing the fuel pump, so I'm unclear about its design or features.)  If you have the single rail system with the remote mount pressure regulator, this regulator doubles as a check valve for easier restarts.  If the system is bleeding down as quickly as you describe, the regulator could be at fault and not holding pressure for a restart.  That sounds like the case from your description.

Until you remedy the check valve (filter/regulator if the later kit) issue, try "priming" the system before cranking the engine.  Turn the key just to the ON position and listen for the pump operation.  The pump should run until the pressure is up.  You may need to recycle the key more than once to reach full pressure.  Once pressure is up, crank the engine over.  It should start immediately.

If there is an overall pressure issue at the fuel rail, even when the engine is running, that could be the regulator or pump.  52 PSI should be enough pump pressure, though, as long as there is sufficient volume flow at that pressure.  (Actual fuel pressure at the fuel rail should be 49.2 PSI +/- 5 PSI on a single rail system, so 52 PSI would be right in there for pressure.)  After confirming pump pressure and volume, I would troubleshoot the pressure regulator's check valve function.

Moses

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