Jeep CJ-7 4.2L With Mopar EFI Conversion: Temperature Sender Question

[60Bubba]

Good evening all.  I'm new to this forum, but I've been jeeping since the 1990's.  I'm on my third copy of the Jeep Owner's Bible, as they tend to not come back when lent out!

 

The current project is a 1983 CJ-7.  It started life with a T-5 and the 258.  At some point, the previous owner installed the complete MOPAR MPI kit.  Earlier this year, just before I bought the Jeep, the 258 bit the dust.  The shop the PO used replaced the block with a junkyard 4.0.  I honestly don't know if they used the MOPAR retrofit MPI kit on the used motor, or if they just bought a full 4.0 used and swapped it over. Bottom line, it was from a 1995 (Cherokee, I think) and has the 7120 head, fuel injection, and the MOPAR CARB exemption stickers (good for California).

 

Everything seems to be working well, but I have one problem I can't quite figure out.  When I bought it, the engine temp guage didn't work at all.  I checked most of the connections, and when I didn't find anything wrong, I just replaced the sender for $6. Now the guage comes alive as the engine heats up, but it almost pegged to the hot side after about 10 minutes of idling.  I am very confident that the engine is in fact running at the correct temperature.

 

I replaced the sender with a CJ-7 258 part, rather than a 4.0 specific part because of the correct stud connection for my CJ wire harness.  Is it possible that the 258 part has different internal resistance than the 4.0L part?  If so, should I get the 4.0L sender and just get the correct connector to adapt to my CJ harness?

 

Again, I'm looking forward to more great knowledge here.  I know there are bigger forums out here, but I don't think there are any with more trustworthy information!

 

Case

[Moses Ludel]

Case, welcome to the forums and thanks for your patronage around the Jeep Owner's Bible...

 

If the previous owner installed a Mopar EFI conversion, it came with a wiring harness, thermostat housing and a temperature sender that mounts in the thermostat housing.  The MPI/EFI conversion was offered by Mopar Performance in two designs.  The original conversion kit is essentially 1994-95 YJ Wrangler (off-the-shelf) two-rail design with an aftermarket inline fuel pump mounted near the fuel tank.  The later kit is patterned after the '97 TJ Wrangler with single rail MPI and uses a modified TJ fuel pump with regulator, mounted externally near the tank.  Both systems use a similar, earlier style PCM (60-Way or pins).

 

The earlier kit is distinct with two rails at the MPI/EFI system and a return line to the fuel tank.  On this system, the pressure regulator is at the fuel injection rail end.  A similar looking device on the later kit is actually a fuel damper and not a regulator.  (This is off-the-shelf '97 Jeep TJ Wrangler.)  The distinction is single- versus two-rail EFI and whether or not there is a return fuel line from the EFI to the fuel tank.

 

So, first determine which system you have, Case.  Mopar supplied each kit with the thermostat housing/sender plus an engine wiring harness.  You should see very clearly that the sender looks like '91-up MPI era and not the CJ or early YJ carbureted 4.2L type.  Both the wiring connector and the ohms rating would be different.  '83 CJ did not have a PCM like these Mopar MPI/EFI systems, and the temp sender was not an integral part of a PCM's warm-up cycle as well as a temp gauge sender.

 

Let us know what you find here.  This should clarify whether you have the Mopar conversion kit wiring harness.  It should look much like either '94/'95 YJ Wrangler or a '97 TJ Wrangler.  If, instead, the previous owner went to lengths swapping an OE harness from an MPI/EFI era model, there would be little likelihood of a Mopar E.O. decal.  The temp sender should match the '94/'95 YJ Wrangler design if the EFI is a Mopar conversion kit.  The kit sender is critical for engine warm-up enrichment and post-enrichment air/fuel mixtures. 

 

If you have a temp sender mounted independent of the thermostat housing (like a stock '83 CJ position), test the sender wire to gauge lead for continuity and shorts.  If the original CJ gauge wire goes to this sender, the sender is likely an OE '83 CJ type sender.  Reading high as the engine warms could be either the ohms setting/defective sender or a ground/short in the sender wiring circuit.  Of course, the gauge itself could also be defective and should be ohms tested if suspect.  If this sounds complicated, think of the temp sender lead as a variable ground.  If you ground out this lead, the gauge will peg.

 

I installed a 4.0L long block in my son-in-law's '87 YJ Wrangler and used the Mopar EFI Conversion Kit.  Underhood, it looks largely like a mid-'90s YJ Wrangler/Cherokee 4.0L MPI engine.  If you look closely, the induction system is TJ era single rail from the later version Mopar EFI kit.  This Jeep passed the referee station engine change (replaced a 2.5L four), the equipment/visual check and the tailpipe smog test with flying colors.  The engine bay has a Mopar E.O. decal in place.

 

Moses

[60Bubba]

Moses,

 

Thanks for the very prompt and thorough reply.  I'll have to get some better pictures later today after work, but I can tell you a few things for now.  First, the system does have two senders: one is in the water pump housing and is attached to a weatherproof two wire plug in the EFI harness.  The second is the CJ style sender with the threaded post terminal.  It is in the head at the rear of the engine on the driver's side.  This is the one hooked up to the CJ gauge.  Currently, this one has a CJ-7 correct sender that I replaced yesterday.  Because it does at least indicate temp, now I was able to disconnect/reconnect it while the engine was warm to verify this is the one wired to the gauge.

 

As for which EFI the Jeep has, I'm 99% sure it is the earlier two rail system, based on what I see on the manifold compared to the many pictures I've seen on the web.  I'm also 99% sure this has the aftermarket harness.  Several of the wires in the big loom in the engine bay running to the EFI PCM have commercially printed labels.  For example, I can see the tach lead wire sticking out of the loom, and it has a label.  This tells me whoever installed the EFI use a commercial harness, rather than a thinned out donor harness.

 

I'm attaching a picture I took before I went out to shop for a replacement air intake hose.  Coincidentally, you can see what I believe are the two EFI rails as well as the water pump-mounted temp sender for the computer.

 

I'm waiting on some fiddly litle replacement fuses for my multimeter (I'm really an amateur with it, hence the blown fuse).  When I get that I can try to check resistance on the water temp gauge.  Based on what you already said, I'm fairly certain I'll solve my problem if I get a correct YJ temp sender for the rear sender mount point, put the correct terminal on it and mate it to my harness for the CJ gauge.  Thanks again, and let me know if you think I'm on the right track.

 

Case

[Moses Ludel]

Case, as you describe, the temp sender circuit should be completely independent of the EFI conversion kit wiring.  Therefore, I'd stick with the OEM single wire sender, not a YJ type but rather an '83 CJ replacement. 

 

For a quick check, try temporarily ground the sender lead while someone watches the gauge.  If you're concerned about frying a wire, use a fused jumper to the ground.  There really should not be much amperage here.  Key on and grounded sender lead, the gauge should immediately scale over to the "Hot" side.  Do not peg the gauge.  This is simply a fast test of the gauge function.

 

Again, think of the sender as a primitive rheostat.  Usually a wax pellet design, the sender takes the engine block ground signal to the gauge in varying increments.  The temperature of the coolant applies to the sender probe, and the ground resistance changes with the fluctuations in temperature.  So, there are two concerns here:  1) proper functioning gauge, which is also resistance based, and 2) the sender sending the right ground resistance measurement to the gauge.

 

If the gauge reads high, it would have me looking for a "leak" to ground somewhere between the sender, sender stud/terminal and wiring to the gauge.  If there is a slight short-to-ground, even excessive debris (conductive) from the sender stud to the shell of the sender (ground), the gauge will be inaccurate.  In your situation, reading "Hot" means too much ground.  With the wire disconnected from the sender and held free of the block or any grounding surface, the gauge should be stone "Cold".

 

If all this testing has no effect or is inconclusive, consider testing the gauge cluster...Keep us posted!

 

Moses

[60Bubba]

I have the 1984-86 FSM, and I just found and read through the troubleshooting for a malfunctioning guage.  Is it safe to assume that the resistance readings for 1984 are pretty much the same as those for my 1983?  I'm going to post some more pics of my engine I took this afternoon, in case those help determine if I do in fact have the two rail system.

[Moses Ludel]

Yes, Case, it's reasonable to assume. 1981-86 is essentially the same. I use an '83 book for all models built to 1986. You should be okay with the '84-up test procedures and specifications.

By the way, you do have a Mopar EFI conversion, which does fit 1981-90 CJ/YJ 4.2L and will also work with a 4.0L '91-up engine. You have the latter at this point. I don't see the return fuel line. Am I overlooking something here? A photo of the pressure and return lines at the rail or the type of fuel filter and fuel pump involved would help clarify.

Moses

[60Bubba]

By the way, I never did find a fuel return line from my fuel rail.  What year was it they went to the returnless fuel injection on the MOPAR kit?  I noticed the CARB exemption stickers are copyrighted 2001, so the kit was at least that new.  Also, as the previous mechanic did the 4.0 swap (1995 Cherokee engine) some time after the MPI upgrade on the old 4.2, I really do have a frankenengine going here.  Was the newer MOPAR kit compatible with the older (7120 casting) intake manifold?

 

Doing my best to sort it all out!

[Moses Ludel]

Case...I hinted that your system looks like the later Mopar EFI Conversion Kit.  This was a change in the fuel supply system and the induction manifold and fuel rail.  You should still have the 60-Way (pin), single plug PCM.

 

I've tried to guess why the parts change, the two-rail system (1991-95 YJ Wrangler) works well.  Likely Mopar no longer wanted to support the 1994-1995 intake manifold, fuel injection rail design and hoses needed for the original kit, which consisted of off-the-shelf 1994/95 YJ Wrangler and XJ Cherokee parts.  The OBD-II rulings may also have influenced this change.  Regardless, think of your system as essentially off-the-shelf 1997-99 TJ Wrangler.  This is handy when you need a sensor or individual components.  Note that there is a "custom" fuel pressure regulator near the tank, mounted externally. 

 

This is actually a modified 1997-up TJ tank regulator that normally fits atop the in-tank fuel pump on a single rail factory EFI system.  The modification makes this a standalone piece that does not mate with the in-tank pump module like a TJ Wrangler.  There is no regulator on the fuel rail, the item that looks like the two-rail regulator is actually a fuel pressure "damper" to stabilize fuel pressure.

 

Your later 4.0L engine accommodates this system readily, I did a similar approach for my son-in-law's '87 YJ Wrangler, using a mid-'90s replacement 4.0L long block with your later Mopar EFI single rail package.  This made the changeover easier.  He's at San Diego County and has passed the referee station inspection and subsequent smog checks.  Needless to say, his engine burns very clean, it should meet 1997-up TJ tailpipe readings with the cat in place.  This beats both the '87 2.5L TBI engine and the 4.2L carbureted engine.

 

A 4.6L build would be practical if you ever rebuild this 4.0L engine.  Beyond the 4.2L crankshaft and right piston/rod combination for correct block deck height, you'd only need to change out the injectors.  I talk about this in depth at the magazine.  (At the magazine Search Box, simply insert the keyword: 4.6L...You'll get plenty of returns links!)  Presumably, with these replacement injectors, the tailpipe readings are within bounds for smog inspections.  I've heard no reports to the contrary.

 

Moses

[60Bubba]

I'm definitely planning on a 4.6L stroker.  I've already watched several of your videos and read everything I can.  I know my engine has at least 155K on the clock, and of course since it came from a junkyard, I have no idea what condition it was in upon installation.  It runs great now, and doesn't seem to burn oil, though I haven't owned it long enough to say conclusively.

 

When I finally grind this engine into the ground, I'm sure I'll be back here looking for advice and further info.