Q&A How-to: Jeep XJ
Cherokee and MJ Comanche—Fuel, Spark & Emissions
4.0L MPI Six 'Banging' Without
From: John A.
Sent: Sunday, August 12, 2012 4:54
Subject: Should I just shoot it?
Help! I have a 91 Comanche 4.0L 5-speed. Have owned for 4+ yrs. 260k miles. Good overall condition. Has
always run great.
Did a full valve job, new lifters, oil pump about 2 yrs ago.
About 2 months ago engine started with an erratic miss. Kept getting
worse, so, I started working on it. She "bucks" terribly at times, mainly when accelerating gently, but can be when
cruising. Makes you feel like it is going to tear the driveshaft out from under the poor thing. Will jerk 2 or 3
times in one second. Not just a skip, but a hard "bang, bang, bang" on the entire drivetrain. You have to depress
clutch fast to keep it from tearing something up. (have checked for a cracked flywheel, seems OK).
Replaced TPS, Idle Motor, Dist Cap, Plugs, Wires, Fuel Filter. No better, took it to
a shop. They said ECM, so replaced that.
Back to shop, 3 mechanics and an analyzer. Smoke test, fuel pressure (35 psi), vacuum
(15"). They have no clue.
Replaced O2 Sensor, Speed Sensor, Intake Temp. Sensor, NO better. Replaced
distributor. Checked all grounds, fuses, relays.
Still doing the same thing. Cooling fan doesn't run either, have it hot
So, any suggestions? Anything will be much appreciated. Built an all-aluminum over
cab topper recently. Am in process of total re-upholstering truck interior. Plan to keep, is my only
Thank you in
From: 4WD Q & A qanda@4WDmechanix.com
Sent: Sunday, August 12, 2012 8:32 PM
To: 'John A.'
Subject: RE: Should I just shoot it?
John, two things come quickly to
1) Have you checked the crankshaft position
sensor at the back of the block (upper converter/bellhousing, 11 o’clock looking from rear of engine)? A dirty
sensor, or defective sensor, will cause these symptoms. Oil from the rear of the engine (main seal, typically) can
even cause the sensor to fault. Check sensor wiring for a short, too.
2) You have fuel pressure…Do you have fuel
volume, too? A clogged tank pickup or other restrictions, or even a defective fuel pressure regulator on your
two-rail system, can cause a problem.
We can take this further if necessary…I’m full
of ideas, just furnish details!
2.5L TBI Issues...
Sent: Friday, November 25, 2011 7:08 PM
Subject: Engine stalls and dies
I have an 86 comanche. it runs for a while then stalls (sputters when applying
throttle) and then dies. turn off key and cranks right backup.
From: 4WD Q &
Sent: Saturday, November 26, 2011 4:01 PM
To: 'James M'
Subject: RE: Engine stalls and dies
James...173 (2.8L) V-6? Sounds like an oxygen sensor or EGR valve issue. If a 2.8L,
you have a carburetor with feedback…Does the engine run okay after the restart or run for a while and stall
Have you changed
the fuel filter lately?
Sent: Saturday, November 26, 2011 4:31 PM
To: 4WD Q & A
Subject: Re: Engine stalls and dies
Fuel filter change was the first thing I did
It is a 2.5L. It runs fine after restart for about a mile then
Connected by DROID on Verizon
To: James M.
Subject: RE: Engine stalls and dies
Date: Sun, 27 Nov 2011 15:13:14 -0800
James, I have a lengthy 2.5L TBI article at the
magazine. Give it a read:
The crankshaft sensor pickup at the flywheel is a common trouble spot, often causing
stall and other issues. Oil seepage or debris can cause trouble at the sensor pickup. I talk about the
device in this article.
From: James M.
Sent: Monday, November 28, 2011 7:31 PM
Subject: RE: Engine stalls
the help. I have see it said to use O2 sensor safe cleaner. Is Sea Foam Deep Creep a good for
also can you
look at the attached pic an help Identify these loose connections.
lines need a circuit diagram to trace out. Do you have a vacuum diagram?
Not sure about
Sea Foam Deep Creep, haven’t used the product. You have a TBI system. Have you read the suggested material at the
magazine site? Here's the link:
TBI 2.5L Engine Tuning and
Engine Performance Quirks
thanks for the great books, Moses, and for answering my questions! To begin, Mopar used to issue a book on Jeep
engines, and I’m not sure how to get a copy. Any idea where to get a copy? I have a 1989 Wrangler YJ with the
mighty TBI 2.5L AMC four!—Brian L.
[Note from Moses—The letter/questions below are from a Jeep YJ
Wrangler owner. The answers also apply to the pre-1991 2.5L four-cylinder engines found in the XJ Cherokee
and MJ Comanche models. XJ and MJ owners will find this useful...]
Brian, the book
is Mopar Jeep Engines, Speed Secrets & Racing Modifications for
Jeep-Built 4, 6 & V-8 Engines, published as Mopar part #P4529529. It covers AMC-design
engines, winning formulas from off-road and compact truck racing of the late ‘80s.
factory workshop manuals and racing data, this is the only official reference to racing these AMC engines. If you
cannot find a copy through used literature sources, secure a copy of the Mopar 1989 Jeep Service Manual, a two-book set that includes your YJ
My 2.5L has non-adjustable, stamped steel
rocker arms. The Federal Mogul stock pushrods for this are 9.486" length, and Clevite's are 9.520". What kind of
lifter preload should I have when it comes to fitment of pushrods? I’m getting preload ranges from 0.085" to
0.100". It is my understanding that lifter preloads should be more in the 0.030”-0.040" range, my reason for
double-checking. The engine is a stock remanufactured long-block from ATK and now has 15K miles on it. The vehicle
has just over 111K miles, and I installed the remanufactured engine at 97K. The camshaft is within a few
thousandths of new specs for lobe height and duration. Questioning local engine machine shops has left me with a
variety of answers, and I'm not too sure which ones to trust. (One shop reassembles everything and doesn't check
preloads.) It seems like this type of engine runs pretty warm although it may be just this particular unit. I have
noticed that after 30 minutes or more of freeway driving, at idle, the oil pressure drops much lower (to 20-25 psi)
than where it usually runs in town (40 psi). Bringing it just off-idle brings pressure back up to around 40 psi. It
appears that this is due to the oil being so hot, as though its viscosity really thinned out temporarily. Once the
motor idles for a while or is off for a bit, it returns and stays at the more normal 40 psi. Taking the vehicle on
extensive or even moderate off-road use has the same effect in increasing the oil temp. When the engine gets this
hot, and I notice the oil pressure, I can also tell that it is much more apt to want to knock/ping if I am doing
something that requires mild lugging at idle. Is this high oil temp scenario typical for the 2.5 liter? Could the
EGR valve and flame temps too high create abnormally high coolant and oil temps? I tried a new EGR valve, which
didn't seem to fix the temps. Should I install an oil cooler? The vehicle sees 90 degrees F to -45 degrees F. If an
oil cooler is called for, which style may perform better: one with an electric fan and thermal switch or an
older-fashioned unit with a built-in thermal valve assembly?
Brian, oil temperature does play a role here.
The "normal" factory-rated oil pressure for a 2.5L four of your vintage is a minimum of 13 psi @ 600 rpm and
maximum of 37-75 psi @ 1600-plus rpm. (Normal at an 800 rpm warm idle speed is 25-35 psi, which your engine may
also achieve.) Despite the drop you describe at idle when hot, this is still adequate for lubing the engine. An
engine oil cooler, unless thermostatically controlled, would be overkill in your cold winter conditions...If you do
either kind of thermal control, make sure that there is no winter issue with oil temps being too cold...Also, if
you want to know how "hot" the oil is actually getting, try using an infrared-sensing thermometer to check the
surface temp at the oil pan after running. You can spot check the engine at the valve cover, block and timing
cover. The block surface temp should pinpoint effectiveness of the cooling system. Check the radiator for hot spots
or lack of flow, using the infrared to track heat…This can be a simple and effective diagnostic tool! As for the
EGR valve, this valve actually lowers the upper cylinder temps if working properly. Its purpose, reducing the NOx
emissions, demands lowering the upper cylinder temps to below 2500-degrees F. Without the EGR working properly,
temps can reach 4800-6000-degrees F, much like an acetylene torch! I always recommend leaving the EGR in place and
making sure it functions correctly…Pushrod lengths, which affect the valve clearance, are critical. (If needed,
adjustable pushrods are available from Clifford Performance for the 4.2L inline six and other AMC-derivative inline
There are two ways to measure lifter clearance:
1) preload, which is essentially zero valve lash with the plunger fully extended then add a fixed amount of
compression of the lifter plunger and 2) lifter plunger travel, measured at the valve stem-to-rocker arm gap, with
the lifter’s plunger fully collapsed. Non-adjustable stamped rockers usually can only be checked for lifter travel
with the plunger collapsed. Correct specs for plunger or valve stem clearance (with the plunger collapsed fully)
should be available through ATK or Clifford. Make sure you measure clearance at the correct reference
point—specified typically as the valve stem-to-rocker arm gap with piston at TDC of its compression stroke, both
valves closed and seated, the hydraulic lifter collapsed (without any oil in the lifter). Specified clearance
should account for the valve rocker arm ratio. (The stock ratio is 1.6:1 for a 1989 2.5L Jeep engine.) The
blueprint build of an AMC inline six or four should include cutting the installed valve stem heights to compensate
for both the cylinder head resurfacing and the amount of valve seat inset from reconditioning the valve seats…A
bled-down clearance of 0.085"-0.100" at the rocker arm-to-stem gap is not out of the ordinary for modern hydraulic
lifters. (Engines like Ford’s ‘FE’ big-block V-8s rely on pushrods in various lengths to correct lifter
clearances.) Mopar manuals do not offer a specification; they assume that the cylinder head has not been surfaced
excessively, pushrod lengths are correct, head gasket thickness is right, and the valve seat depths and valve stem
heights are correct. It’s wise to confirm this and check actual bled-down lifter clearance if possible...Engines
need adequate valve clearance to permit valves to seat. Shorter Federal-Mogul pushrods may be for remanufactured
engines with a surfaced cylinder head and ground valves and seats. If your 0.085”-0.100” is clearance measured at
valve stems-to-rocker arms with the lifter plungers collapsed, note that Ford ‘FE’ V-8s (with a 1.73:1 rocker ratio
and non-adjustable rocker arms) call for 0.100”-0.150” gap at the valve stem-to-rocker arm with the lifter plunger
collapsed. (Ford FE pushrods come in different lengths for adjusting valve clearance.)
Brian’s 2.5L tuning
I have found on this vehicle a normally
energized-closed solenoid valve that controls vacuum to both the EGR valve and the fuel vapor charcoal canister.
When I did the engine change, I put on a new EGR valve and replaced the rest of the engine sensors. Jeep
discontinued supplying the G.M. Rochester vapor canister; it appeared to be intact. Recent troubleshooting led me
to the canister's vacuum-operated control diaphragm. It turns out that this little rubber diaphragm has holes in it
from being 18 years old. With a breach in the diaphragm, the vacuum that is constantly applied at the canister
purge port can leak through and back-feed through the diaphragm to the canister's purge signal port. Instead of the
EGR solenoid valve controlling the vacuum to the canister's purge signal port, there was vacuum continuously coming
from it. This was no real problem except that the EGR valve is also tee’d into the line for the signal port. When
the canister back-fed vacuum, it constantly supplied vacuum to the EGR valve. This kept the EGR valve always on,
during every aspect of driving. I don't know how long this went on, but when I recently tested my EGR valve, it had
failed. I have replaced it and now am shopping for a replacement canister. Even with the canister's interlink to
the EGR valve disabled so that the EGR solenoid controlled just the EGR and not the vapor canister, and even with a
new Mopar EGR valve, the engine oil temp still gets so hot that it operates as I’ve
Brian, the EGR open at idle will dilute the mix
and confuse the oxygen sensor into leaning out the air/fuel mixture...Such a lean mix can cause engine
heat-up...This could cause the oil temp to rise, too. The EGR is a valuable device that lowers upper cylinder
temperatures. It is also passive and has little impact on performance with the 2.5L four.—Moses
added this question…
question that I don’t believe is related: This motor has vacuum fluctuations from 20"/hg to around 15"/hg. I can
hear the idle hunt a bit, and see it on the tach—up to 200 rpm fluctuation at times, sometimes barely any. Any
vacuum gauge sits there and flutters very fast. The vacuum flutter seems to narrow when the engine rpm is brought
up, yet it still does it to some degree. After going through every vacuum line on the engine to hunt for possible
leaks (hence finding the charcoal canister to be bad) and after double-checking intake and exhaust manifolds (the
engine pipe and rest of the exhaust system are all new), I am unable to figure this one out. I have verified all my
valvetrain to the point of degree’ing everything out, removing valve springs and checking stems-to-guides for any
excessive wear, etc. Do I need to remove my exhaust manifold and check for cracks, as the factory manifolds love to
crack at the Y's, which are hidden by the preheat ducting? Is it possible that I am seeing the vacuum flutter from
the power impulses since it's just a 4 cylinder, whereas a 6 or 8 might be smoother? Would some small cracks in the
exhaust manifold have any effect on intake vacuum?
power pulses should not be a factor. Vacuum flutter can be valves that do not seat completely. This can cause
engine heat-up as well. Pushrod length and/or valve stem length control the lifter clearance on the AMC fours and
sixes. The rebuilder must restore lifter clearance, and if valve clearance is too close, valves will not seat,
which worsens at higher engine temps...This can cause heat buildup and eventually burn valves and
The EGR not
seating at idle/low speeds also causes a vacuum drop or vacuum fluctuation. Lean fuel mix, air leaks, ignition
timing errors and degrees of spark advance will also affect vacuum. Ignition timing is electronically controlled on
your engine, with sensors feeding input to the module. The oxy-sensor "averages" oxygen readings from all four
cylinders and adjusts the air/fuel ratio accordingly. 2.5L fours are reasonably smooth by nature and shouldn't
pulse or surge dramatically. This, again, points to dilution from the EGR or a vacuum leak, which can also be
caused by an EGR that does not seat properly at idle. Make sure that the throttle position sensor is working
properly and giving the right voltage readings from idle through open throttle...A defective TPS can cause idle
roughness, fuel mix problems and tip-in issues.
ruled out valve timing and lift issues, the best diagnostic steps now would be a cylinder leakdown test (engine
still, with each piston at TDC of its compression stroke). This will pinpoint any valve leakage or ring seal
losses. Also check for intake manifold-to-cylinder head leaks and seepage at the throttle body base and shaft.
Engine at idle, use a non-volatile penetrant spray; leaks will show up as a change in engine
Waiting to hear
how this turns out!