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Cherokee XJ RENIX 1990 - low power, bad mpg, problem with ignition and timing

szachmat

NAXJA Forum User
Location
Poland
Hello,


I'm Mat, im from Poland and I have a problem with my Jeep Cherokee XJ 4.0 1990, renix.

It's on 31 inch tires and 3 inch lift and i have like 16 seconds for 0-60mph, it's hard to pass 75mph, and 85 mph is impossible to reach, and with normal driving on the streets i get about ~7 mpg. Highway cruising gives me 9mpg.






I have replaced O2 sensor, TPS sensor, MAP sensor, engine timing chain and sprockets, distributor cap, spark plugs, wires, ignition coil (all brand new), and i tried new ignition module (got used one from an old renault).

I have checked temperature sensors (engine coolant and air temperature), and CPS they are fine.


I have checked my timing with a strobe lamp (with regulation) and its about 52 degrees advanced. No matter what i do with a distributor, or any other thing its always on 52~degrees. I tried to set a distributor in other position but it doesnt change anything, jeep starts but its still about 50 degrees advanced.


I order complete new distributor and CPS sensor but i need to wait 2 to 4 weeks till it will arrive from US, because its impossible to get one in Poland, but im not sure if it can help....


Im running out of ideas and my jeep still isn't working as i would like.
 
Timing is regulated by the ECU (computer) and the CPS (crank shaft position sensor)

Unless the transmission has been off and the flywheel/flexplate installed wrong (hard to do due to bolt spacing) I would say the problem has to be the ECU.

The CPS either reads the notches on the flywheel or doesn't. It either runs or it doesn't. The notches are in a particular order and the computer looks for this order before supplying the spark. The odds of the CPS causing this are one in a million.
 
My suggestion to you is first visit Cruiser54's website and read what he has written. Randomly throwing parts at your Renix XJ is not going to solve your problem.

Best regards,

CJR
 
Thank you for fast replay!


You are talking about ECU under main console inside car?
I always heared that its regulated by ignition module (the one with coil on it)
 
@CJR, thanks for tip, but i have changed all those parts because my jeep had other problems too, it eliminated most of them.
 
I have checked my timing with a strobe lamp (with regulation) and its about 52 degrees advanced. No matter what i do with a distributor, or any other thing its always on 52~degrees. I tried to set a distributor in other position but it doesn't change anything, jeep starts but its still about 50 degrees advanced.

This makes no sense, as in it's basically impossible!
 
I too have a 1990, sluggish performance, finally figured it out, the catalyst was breaking down and clogging. Test by removing the cat. or put a vacuum gage on the intake and see what you get. I had to diagnois mine on a road trip, so I had no vacuum gage, so I unbolted the cat. , a vacuum gage will also give clues to valve operation problems. also check for intake leaks, that is a quick and easy test, and will eliminate leaks as a problem if it passes.
 
Guys, yesterday i tried to clean some grounds, connectors and tried few things, and now i', totaly confused....

So i unplugged spark plug on a 1st cylinder, to check if TDC really matches with a mark on vibration damper, and.... COMPLETELY NOT.
So i put engine on tdc on 1st cylinder, and re indexed distributor so now it maches #1 tdc as it should, and it changed nothing. Power is still the same ~15-16 sec 0-60mph.
But i found another funny thing. Accidently i didnt plug in my distributor sensor, and jeep started normally. I tried to unplug the distributor on running engine and it makes no changes. It completely doesn't matter if its plugged or not. Then i checked the voltage on connector. According to renix manual i have found it should be ~5 volts. I have 2.91volts. I tried to take new ground from a chassis, and when i do it's then 7.91volts....
Does that mean i need to look for a damaged cables in my instalation?
 
It is not uncommon on the older vehicles that the rubber in the harmonic balancer degrades and the pulley slips. While that will not really cause the timing to be off, it will make it look like it is.

The old school method to find the real TDC is to put a rod down in the cylinder through the spark plug hole. SLOWLY rotate the crank with a wrench until the piston softly contacts the rod. Then mark a spot on the harmonic damper. Then rotate the crank the other direction until the piston once again hits the rod. Mark that spot on the harmonic balancer. True TDC should be half way between the two marks.
 
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It is not uncommon on the older vehicles that the rubber in the harmonic balancer degrades and the pulley slips. While that will not really cause the timing to be off, it will make it look like it is.

The old school method to find the real TDC is to put a rod down in the cylinder through the spark plug hole. SLOWLY rotate the crank with a wrench until the piston softly contacts the rod. Then mark a spot on the harmonic damper. Then rotate the crank the other direction until the piston once again hits the rod. Mark that spot on the harmonic balancer. True TDC should be half way between the two marks.

You beat me to it!!!
 
That's how i did it.
But still i don't know what to think about my dristributor and it's voltage.
And why i have so bad mpg and low power.
 
But i found another funny thing. Accidently i didnt plug in my distributor sensor, and jeep started normally. I tried to unplug the distributor on running engine and it makes no changes.


On the Renix, the distributor sensor is only used to determine which stroke it's on because it can't know that by just looking at the crank sensor. If there is no dist sensor input when it first cranks, the ECU will take a 50-50 guess. I don't think it should have 5-volts to the sensor, but not sure. I doubt the ECM is the issue.



Spitballing a bit... How did the old plugs look? All similar, one significantly cleaner or darker than the rest? I'm just wondering if you've got a leaking or failed injector throwing the O2 sensor off and making all the other cylinder run rich. Does the exhaust smell rich? A set of refurbished 4-hole Bosch sensors are relatively cheap, btw. Do you have power to the O2 sensor heating element? Is it the correct O2 sensor, as the Renix used a different type than HO? Is the vacuum line to the fuel pressure regulator intact, and does the fuel pressure look correct at idle and hold when the ignition is turned off? Pull off the vac line to the fuel regulator and make sure it's not leaking gas into the vac line. Compression numbers look good?



Definitely visit Cruisers site and start walking through all the tips.
 
I have dual fuel system in a car, gasoline + lpg (propane butane) and there is no difference so i don't think its bad injectors.
O2 sensor is correct 5v for renix. I didn't check the voltage on it but when i replaced the old one, i felt some difference on +.
The old plugs were all black. All the same.
I need to check the vaccum on the fuel pressure regulator but i checked few vacuums before and they were all good.
Compression rate were all the same ~about 10 bar on each cylinder.
I didn't check fuel pressure. Fuel pump is 1 year old, but as i said i have also LPG system and it makes no difference.
I was also using an AFR sensor and the mixture was perfect. I checked the emmission too and they told me its all good. I can check again and show you results.
Exhaust smells a bit rich, but i have no catlyst.
 
Maybe its metric degrees instead of English degrees. :gag:
 
No, it did not match.
TDC mark on vibr. damper doest not match with real TDC.
I think it somehow moved on that rubber between two parts of that wheel.

Now i set my TDC by checking the real position of a piston (like in the instruction in post before) and re-indexed my distributor.

No matter what i do, what i change, if it's indexed correctly or not, it still gives same results :S
Not sure about actual mpg, but if i get no power back i assume mpg didnt improve.
 
A couple of things to look at, the oxygen sensor, knock sensor and engine temp sensor harness runs up the front of the engine. If routed wrong it can cook on the exhaust manifold.

Two holes in the throttle body for the MAP sensor vacuum line. Only one home goes all the way through to vacuum. The vacuum tube for the MAP is prone to rub through or crack, it only takes a tiny flaw to cause issues.

Do an ohm test on the TPS ground. That ground circuit has many splices any of which can cause issues.

Jump the ballast resistor.
 
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