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Diesel Sound when Accelerating

Rusty XJ

Rusty XJ

NAXJA Forum User
Location
Texas?
I've read a little bit about similar symptoms but was unable to really relate any of them to my problem...

I have an 88 XJ 4.0 w/ AW4 and recently it has begun making that diesel sound under moderate to hard acceleration... It seems to do it on down shift at around 1500-2200 RPMs or so but goes away at up shift. I can romp on it at a stand still or doing 50MPH and it does the same thing... hot or cold, doesn't matter...

When the engine gets hot if I really get after it it'll burn a little oil... am running 33's and 4.56's and am getting 16-17 MPG on the highway (70-75MPH on 250 plus mile trip)... don't know if that's normal or not...

Any ideas?
 
It's call detonation and it's damaging to your engine, your timing is too far advanced or your fuel octane is too low, start with adjusting the timing and get some good gas, the engine should use 87 octane but try 91 octane and see what happens
 
jerrywagner said:
It's call detonation and it's damaging to your engine, your timing is too far advanced or your fuel octane is too low, start with adjusting the timing and get some good gas,
Click to expand...

It could be knock as one possibility, certainly. The question is what's causing it.

You could have a manifold and/or exhaust gasket on the way out. I've had this happen to me on non-XJs before, and the sound is not dissimilar to a diesel at times. Also pull the spark plugs and check their condition; it's possible you've got one (or more) on its way out. The TPS could be acting up as well. Finally check the condition of the distributor (the whole thing, not just the cap), rotor arm, and wires.

the engine should use 87 octane but try 91 octane and see what happens
Click to expand...

Don't use 91. I think I see why you're suggesting this in relation to the timing problem, but unless the timing's minutely off this won't make any difference. Run through a tank of 87 from a different gas station (it's possible their last batch had dirt or condensation in it) and see what happens.
 
I always run 87 octane. It's been doing this for more than just a couple of tanks of gas... so, I don't think it's fuel related...

I was curious if it could be exhaust related... It's been awhile since I've checked the manifold.

I also noticed the other day that the wires that lead into the TPS have seen better days. Apparently whoever owned this before me didn't have enough sense not to splice the wires when checking it out. They're still in tact, but some of the shielding is gone... I know TPS are common prob's on XJ's, but wasn't sure if it would cause this problem.

I may have a friend who's got one I can borrow for T&E. I'll check the exhaust manifold tonight after it's cooled down a bit.

I'll let ya' know.

Thanks guys.
 
Check your Knock Sensor! Mine sometimes comes loose and causes this exact thing. if it's bad it will mess up the timing!
It will be timing related, but that is controlled by the ecu, and is not adjustable.
The sensor is located on the driver side, under the intake and exhaust manifolds. Follow the wiring that runs from the injectors down that side. There are 3 sensors hooked up to it. The CTS, O2 sensor, and the Knock sensor. The knock sensor is in the middle of the 3.
 
jerrywagner said:
It's call detonation and it's damaging to your engine, your timing is too far advanced or your fuel octane is too low, start with adjusting the timing and get some good gas, the engine should use 87 octane but try 91 octane and see what happens
Click to expand...


I didn't think you could adjust the timing on these... isn't it all maintained by the computer? What else might effect the timing... or seemingly so... MAP? 02? Just thinkin' about other things...

Thanks again.
 
TRAILREADYXJ said:
Check your Knock Sensor! Mine sometimes comes loose and causes this exact thing. if it's bad it will mess up the timing!
It will be timing related, but that is controlled by the ecu, and is not adjustable.
The sensor is located on the driver side, under the intake and exhaust manifolds. Follow the wiring that runs from the injectors down that side. There are 3 sensors hooked up to it. The CTS, O2 sensor, and the Knock sensor. The knock sensor is in the middle of the 3.
Click to expand...

Must have been typing at the same time. *LOL*

Thanks. Will check that!
 
Take a good look at the harness where it continues from the front of the fuel rail down the front of the motor and underneath to the engine (ECU) temp. sensor, the knock sensor and the O2 sensor. This harness often gets cooked on the exhaust manifold, especially if the intake/exhaust manifold or the steering pump has been removed, it often gets reinstalled/routed wrong.
Take a look at your plugs. If they are burning white at idle you probably have a fuel problem. Lean burn or timing are two possible answers.
Does the motor ever buck or have a heavy miss?
 
8Mud said:
Does the motor ever buck or have a heavy miss?
Click to expand...

Nope. Never. Drives like a champ, right the way I would expect it to... kinda figured something may have been vibrated/knocked loose on the trails sometime... but wasn't really sure what to look for.
 
TXHillsboy said:
What else might effect the timing...
Click to expand...

Worn distributor drive. Even though the timing's computer-controlled, the worn drive can knock the rotor out by a few degrees - just enough to screw with when the spark gets sent. Think of it as an involuntarily timing adjustment ;)
 
Plastic around Knock sensor was cracked and broken in several places and plugs were corroded... Will have a new one tomorrow night and let you know if it helped.
 
Alright guys... I'm a dummy... anyone got a picture of this knock sensor in reference to the other two sensors... 'cos when I'm looking at the underside of my Jeep all I see is two sensors... there's the one on the exhaust (02) and there's one right in the middle of the block just above the oil pan. It's cracked and broken and coroded (see previous message) and I would assume it's the knock sensor... but everything I've read says there's 3 sensors down there and the KS is in the middle... like I said, I only find two.

That and the new knock sensor I ordered doesn't look exactly like the old one... (that I'm assuming is the knock sensor)... haven't had a chance to check the electrical connector yet.

Thanks.
 
The knock sensor has a connector that looks just like a fuel injector connector.
m8chno.gif


I looked for a picture of the whole side of the motor and/or the ECU temp. sensor and came up blank. The Temp. sensor I'm pretty sure has a two pin connector, the same as the MAT sensor on the intake manifold. It is just above the motor mount on the drivers side of the block.
The knock sensor is just to the rear of the motor mount and near the pan seam.
 
Yep... that's the one that's buggered up. Any idea if there's a torque requirement on that thing?

Thanks!
 
Last edited:
Well, replaced the knock sensor today after work and the problem is still there...

A buddy of mine who happens to be a fellow XJ owner says it sounds like the main bearing ...

Still haven't checked distributor yet...
 
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