What is the timing?

Have any of you used a timing light on their XJ? What should it be with engine hot in park at idle?

I need to do that at some point. Maybe try one of these discount parts stores for a loaner timing light. Why is a timing light no longer included in a technician's tool case?

MY XJ have intermittent pinging at some load and speed. Mostly, it does not happen, so I tend to ignore it. It is there at idle. Off idle it gone. Between 10 and 40 MPH just around shifting (AW4) it is loud. Step on the pedal it gone. At cruising speed 50 to 80 MPH it is nonexistent. I do not remember when it started. I do remember pulling the distributor some time ago to replace the sync pickup.

It starts first time every time. Returns great fuel mileage for an XJ with a 4.0L. And it runs lean. The only thing I have not replaced in recent months is the oxygen sensor. It is not my DD for about 18 months. I have no complaints about the overall performance although I have never put it to a test.

Let's hear you experiences.

:repair:

Timing is not adjustable so no need to use a timing light for ignition timing.

Check your motor mounts. See if the mech fan is hitting the shroud. How's your oil pressure?

Ignition timing is controlled entirely by the ECU, and there are no user-adjustable knobs associated with setting ignition timing. If you were to put a timing light on the motor, you'd see timing bouncing around because the ECU uses ignition timing (along with the idle air flow valve) to control idle speed.

If you want to see ignition timing, hook up an ODB reader that will report engine timing.

I dont even have a distributor! So yes the PCM is the only way to check it.

96 would have ignition wires. Not sure what to look for if a timing light was put on it though. I do have one. A few years back, I was at my neighbors. Another neighbor had his Chevy Dually in there. I don't remember what for. They needed a timing light. The one said he'd bring his home from work. I said I have one. They both said, why do you have one. I said '75 Bricklin.' 'Oh.' I'll have to check and see if I can find my '98 Manual. I'd wonder about a stretched timing chain or a worn distributor bushing.

Saudade said:

Timing is not adjustable so no need to use a timing light for ignition timing.

Check your motor mounts. See if the mech fan is hitting the shroud. How's your oil pressure?

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Engine mounts and transmission mounts are in perfect shape. There are no mechanical fans on my XJ. Dual electric fans are being used for some 20 years now.

maxbraketorque said:

Ignition timing is controlled entirely by the ECU, and there are no user-adjustable knobs associated with setting ignition timing. If you were to put a timing light on the motor, you'd see timing bouncing around because the ECU uses ignition timing (along with the idle air flow valve) to control idle speed.

If you want to see ignition timing, hook up an ODB reader that will report engine timing.

Click to expand...

Yes, I realize the timing is controlled by the ECM which adjusts it based on inputs from multiple sensors. Using a timing light will however give a baseline. One would be able to see if it is too far advance or retarded. My feeling is the distributer is advanced 1 tooth. How do I verify that? :dunno:

I have seen somewhere the baseline number of 9 degrees BTDC. I guess an ODB scanner is in order. Tomorrow I will retard the distributer 1 tooth and see if the pinning goes away. Tonight, I will spend some time with the manual. Maybe something may be hidden in there that could provide a clue. :read:

techno1154 said:

I have seen somewhere the baseline number of 9 degrees BTDC. I guess an ODB scanner is in order. Tomorrow I will retard the distributer 1 tooth and see if the pinning goes away. Tonight, I will spend some time with the manual. Maybe something may be hidden in there that could provide a clue. :read:

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That's "almost" impossible since each tooth of the dist drive gear represents 27.69* or (13.8* crank degrees). You should see if the dist is anywhere close to being in sync with the CPS.

You would absolutely know if the dist was a tooth off. It would either not start or run extremely poorly.

The engine will run perfect regardless of the drive gear position "if" the distributor sensor is in sync with the CPS.

techno1154 said:

... Yes, I realize the timing is controlled by the ECM which adjusts it based on inputs from multiple sensors. Using a timing light will however give a baseline. One would be able to see if it is too far advance or retarded. My feeling is the distributer is advanced 1 tooth. How do I verify that? :dunno: ...

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If you have a timing light then:

1) If the accessory belt pulley has no TDC notch, then rotate the motor to TDC compression for cylinder #1 as best as possible and mark the accessory belt at the point corresponding to 0 deg on the engine block.

2) Hook a timing light to cylinder #1 spark plug wire and monitor timing.

I'd simultaneously monitor timing by ODB.

Another comment I have is that ignition timing during idle is typically kept several degrees off of optimum so that changes in ignition timing have a strong effect on combustion efficiency which makes it useful for controlling idle engine speed. I wouldn't be surprised the baseline ignition timing for a full temperature motor was between 5-10 deg BTDC.

Also, if your distributor was off by one tooth in either direction, it would be a lot harder for the ECU to control engine idle speed, so I would expect idle to hunt a lot more.

OP, I would follow-up with the timing but I'd be more suspect of a vacuum leak, injectors, and/or O2 sensors.

And carbon buildup.

OP, how high do you rev that 4.0?

RCP Phx said:

That's "almost" impossible since each tooth of the dist drive gear represents 27.69* or (13.8* crank degrees). You should see if the dist is anywhere close to being in sync with the CPS.

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In reality, 13 to 14 crank shaft degrees is not very far off. In my amateur power boat racing days, 13 degrees initial timing was common with total vacuum timing advance up to 25 degrees. they were Chevy blocks starting life as 350 or 454 CI. We used to turn those motors at 6,000 RPM. Carburetors and points and condenser are in a different world to the modern PCM. I will give that a second look.

maxbraketorque said:

If you have a timing light then:

1) If the accessory belt pulley has no TDC notch, then rotate the motor to TDC compression for cylinder #1 as best as possible and mark the accessory belt at the point corresponding to 0 deg on the engine block.

2) Hook a timing light to cylinder #1 spark plug wire and monitor timing.

I'd simultaneously monitor timing by ODB.

Another comment I have is that ignition timing during idle is typically kept several degrees off of optimum so that changes in ignition timing have a strong effect on combustion efficiency which makes it useful for controlling idle engine speed. I wouldn't be surprised the baseline ignition timing for a full temperature motor was between 5-10 deg BTDC.

Also, if your distributor was off by one tooth in either direction, it would be a lot harder for the ECU to control engine idle speed, so I would expect idle to hunt a lot more.

Click to expand...

The idle is always smooth but the RPM could be anywhere from 500 to 1,000.

cruiser54 said:

And carbon buildup.

OP, how high do you rev that 4.0?

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Most of the noise is at idle or just before the AW4 shifts to the next higher gear 1 to 2 to 3.
That 4.0L hardly get above 3,000 since I could run at 80 MPH all day at that RPM. Above 3,000 is for passing slower vehicles only.

I get some grate responses. I just have to get in there and look closely at the usual suspects.
Spark plugs, oxygen sensors, and injectors are over 200,000 miles. All other engine sensors were replaced about 1 year ago which includes the distributor pickup and ignition coil.

Who have done plug reading while trying to tune a Holley 1050 on a 500 CI big block. I need to go back to old school diagnostic.

First, I need to go get another spool of wire to finish welding the brackets on the JK front D44.

White porcelain on well used plugs=bad lean condition. White specks on colored porcelain on well used plugs= detonation and burning of pistons or valves