WilliamTK1974

Hello everyone,

In the last few days, I've heard what I think is pinging coming from the engine of my 1996 XJ 4.0. The vehicle is stock wih 288k miles. It seems to be worse at times when it's under a load like the sort of load experienced when going up a moderate hill at low speed. But here's the weirdest part: it only seems to happen when I have the a/c running. Otherwise, I hear little or no pinging.

I replaced the fuel filter last fall. A couple of weeks ago, I replaced the evaporative purge valve near the back of the engine as the test I ran showed it was out of spec and the vehicle was having some hard start issues. Changing that part helped some. I also replaced the throttle body gasket. The vacuum lines are all good and I have fixed several of them before. Cruise control will hold highway speeds going uphill and the a/c controls work the way they're supposed to. No shifting to heat or defrost at random times.

I've been told that pinging can be a sign of a lean mixture, which might point toward the MAP sensor on the throttle body. I took it off to have a look. The rubber elbow has no leaks and still makes firm contact with the ports. Can the port for the MAP get clogged with the grime that can affect the butterfly and the IAC port? One of the lines from the purge valve runs into the throttle body right below where the MAP sensor fastens on, and I can imagine that might bring some grime in over time.

The vehicle doesn't idle too rough, but it's not a smooth idle either. No Check Engine codes. I don't recall any of this happening til I replaced the purge valve, but I could be wrong.

Any advice would be appreciated.

Thank you,
-William

agreen

MAP sensor isn't likely, but it is possible. Get a scanner that can read livestream data. The ELM327 Bluetooth scanners work well for this. They're cheap and the apps are free.

Are you certain it's pinging?

WilliamTK1974

I really don't know. The cabin isn't very quiet, so there's quite a bit of noise. What I'm hearing sounds like it could be pinging in ping-possible situations, but given the noise floor, I can't tell for sure. I just don't want to melt a piston or anything like that.

Thank you,
-Bill

tjwalker

If it ONLY happens when the A.C is on and this is important to define so you may need some more seat time to determine this, you need to spend your time scrutinizing the components of the A.C. system. Got to narrow down your focus, otherwise you can end up chasing your tail.

But if you think it truly is engine related and happens when the A.C. is off, for a quick and dirty test........try running a tank of higher octane gas. If the pinging goes away with higher octane gas, you likely have a case of excessive carbon buildup inside the combustion chambers.

The 4.0 engine in your 96 XJ (Post Renix years) don't have a "knock sensor" installed so that should be out of play here.

Good luck and keep us update!

agreen

Yep. With an 8.8:1 CR, it's unlikely that you are experiencing detonation. Not impossible, just unlikely. Pull your spark plugs and have a look at them. See if one looks different than the others. Check the gap, replace if necessary. If you're sure it's preignition, try a can of seafoam or berryman to remove some carbon buildup as stated above.

I'm equally as curious as tjwalker is about the AC comment. You said it almost only happens with the AC on. Which does leave that statement open to "it still does happen with the AC off".

Definitely get the scan tool. You can take datalogs of what the O2 sensor is reading, which can tell a volume about what's actually going on.

If you feel like you're going to damage something, don't drive it. Get it fixed before it costs you more!

WilliamTK1974

After driving it last night, I'm starting to think that this isn't detonation, but maybe the beginnings of a squeaking alternator. The sound gets into the cabin more clearly when the a/c is on. The alternator is another story of frustration for another day.

I did go ahead and shoot some Seafoam spray through the MAP sensor port in the throttle body and the port below the throttle body where the line comes in from the purge valve. Not enough to soak everything, but enough to clean out anything that might have been in there. Maybe it helped and maybe it didn't, but the vehicle seems to start faster now and idle more smoothly.

agreen

Check your crank pulley. The pulley is set in rubber on the hub, and will rotate over time. With the added load, I wouldn't be surprised if it's the squeak/ping noise you're hearing.

WilliamTK1974

From a visual inspection from up underneath, I can see that the strip of visible rubber has several cracks in it. I can't twist the pulley or pull it front to back, though.

agreen

Put a paint mark on either side of the rubber and see if it's still lined up after driving with the AC on.

WilliamTK1974

I went ahead and did the swap. Once I got the old one off, it looked like there were some places where it might have shown signs of separation between the rubber and the metal.

Putting the new one on was almost harder than getting the old one off. I did it from underneath without removing the radiator. Still has a squeak in the alternator. That's frustrating because I swapped it less than a year ago.