rheacox

Been having this issue for quite a while (2014). I have a 98 4.0 Sport 5spd with 213k miles that likes to ping under load at high RPM. Really only happens climbing steep hills or heavy acceleration on the highway above 3k.

About a year ago, I switched to 4 hole injectors thinking that after 200k the injectors could be getting tired and somehow causing the issue. That only made the problem worse and I switched back to stock.

Since I've had the Jeep I've been running seafoam through it occasionally with no noticeable result. I did run a can of MOPAR combustion chamber cleaner not long after switching back to the stock injectors and that did seem to make a the pinging a little better.

Now, this only happens on 87 octane. With 93 or even 89 there is no pinging.

All of this seems to point towards carbon buildup but could that really be it??

The engine seems to have been maintained very well for it's whole life. Still makes good oil pressure and when I changed the valve cover gasket the top end of the engine looked almost new it was so clean. It may be flawed logic but if the rest of the engine is in good shape why would the combustion chambers be full of deposits?

Compression numbers are also good and blowby is minimal.

The only other thing I could think of is that there is a malfunctioning sensor causing the engine to run lean but it seems to run nicely apart from the pinging (starts well, idles nicely etc.).

I'm out of ideas.

IJM

Normally my advice for this sort of thing is to dump in a can of fuel system cleaner, fill it up with 93, and take it out for an Italian tune-up (lots of full throttle at high revs). However, it would be good to check your ignition and fuel systems for proper operation first.

rheacox

I'm also an advocate of the Italian tune-up and have done quite a few at this point.

I also agree that more testing needs to be done. What ignition and fuel system tests can I do with nothing more than a multi-meter? I have been looking for an excuse to buy an OBD scanner that has a live feed...

Some other info that may be helpful. The fuel pump is a Carter unit I replaced in 2015 to fix a bad sender (I hate that the whole unit needed to be changed but that's another subject). I also did a basic tune-up, plugs, wires, cap and rotor in 2015. Pretty sure the plugs were Autolite 985's and I checked that the gaps were 0.035".

Been a few years now but I'm reasonably sure that it was pinging the same before and after both of those jobs.

rheacox

Also, I remember the car talk guys mention a few times that a faulty crank position sensor could cause ping but I've always been under the assumption that if the cps is bad the engine simply won't run.

Turbo X_J

89 octane is probably cheaper than a new motor.

rheacox

Agreed, and I don't have a problem using 89. But the issue is that the engine shouldn't need it in the first place.

Does anyone else need to use higher octane gas with a bone stock 4.0?

IJM

Now that you mention it, I do notice some light knock at extended WOT at 3.5k+ RPM. I just figured any engine without knock sensors (I'm assuming the 4.0 doesn't have them) will do this occasionally under high load. Maybe I need to do an Italian tune-up myself.

Turbo X_J

Caish

sounds like just crappy winter blend gas, ethonal.
just run mid grade and go on.
Under load lugging my renix will ping too, but it doesn't have the knock sensor hooked up, its broke and $50 for a new one, screw it.
HO didn't have a knock sensor.
Could do the seafoam thring pouring it down the throttle body and not just adding to the fuel.

rheacox

Done the seafoam down the throttle body, and the water decarbonizing with no noticeable results. Could be crappy winter gas but it pings year round.... I know running higher octane gas will fix the issue but there shouldn't be an issue in the first place.

For some reason I keep thinking that some sensor is causing the computer to advance the ignition too much or not retard it under load.

Cummins93

Perform a warm compression test and leak down test. Engines can increase compression over time. If it increases too much you can experience pre-ignition.

If you didn't personally install the spark plugs pull them out and verify that they are the correct heat range.

Finally verify fuel trims. If you see high short term or high long term your ecu is compensating for a perceived lean condition.

If none of this works, just for fun, disconnect the upstream o2 sensor and see if you can get a knock.

rheacox

I'll have to order an OBD II reader with live data to check the fuel trims and when we get a nice day that isn't freezing I'll check the compression.

I did check the compression around March of last year and it seemed good (160-170 I think) but I guess I didn't do it correctly. Apparently you only crank it 3 times and take that number, I gave it 5 cranks per cylinder.

Also, I can't be sure if this is related but occasionally while sitting at a stop sign with a warm but not fully warmed up engine the revs will drop from an idle as if the engine is under load from 800ish to just below 500 and come back like the A/C compressor is kicking on but it isn't. Happens with the A/C on or off.

This seems to happen more often when coming to a quick stop but it can also happen sitting in traffic.... My first thought was that the float wasn't adjusted right in the carb but then I realized it was 2017 and this wasn't my Model A.

Like I said, I'm not sure if these two issues are related or not but other peoples input never hurts.

rheacox

Well, time for an update.

First, my downstream O2 sensor came unplugged and I was convinced that made it run better. Then I read up on what that O2 sensor actually does and that it really doesn't affect the engine operation at all... back to square one.

Then I started reading up on spark plugs and some people seem to think that the factory plugs on later 4.0s are too hot and will cause ping without any other engine issues.

I still don't buy that since I know 6 other people with 97+ XJs that don't ping and definitely have stock plugs...

With that said though, swapping from Autolite 985 to Champion RC9YC eliminated the pinging. I'm happy but still convinced theres something else but I haven't gotten around to buying an OBDII reader with live data to check the fuel trims.

mannydantyla

Colder spark plugs can help with pinging but it could be masking a dangerous problem like a lean condition. But if there is no underlying problem then maybe it's an appropriate solution. Maybe it's the modern gas these days, or even the "summer blend". idk

Have you checked the fuel pressure? Maybe the pump is getting weak or the filter is getting clogged and causing low fuel pressure when the engine needs it most. This would cause a lean condition which would cause pinging.

Do you have a scan gauge tool? I got one online for like $20, it sends out a wifi signal that a smart phone app uses to tell you all sorts of diagnostic stuff and dynamic parameters. One that I have found helpful is the upstream o2 sensor. You can cruise at the high speed, going up hills, and look at the o2 voltage data. It should be running on the rich side when under a load, to fight pinging and overheating. It should oscillate 0.7 volts at that speed. At least, that's what mine does. Check it out: https://www.cherokeeforum.com/f2/wha...9/#post3500415

A cheaper way to test for lean condition is to look at the spark plugs. But because you're pinging at heavy loads, it will be harder to get an accurate test for that condition. You could floor it up a hill and as soon as you get to the top, turn off the car. Then pull a plug and get a reading. That's what I had to do with my motorcycle engine to find the right carb jet sizes.

rheacox

I agree that the colder plugs are probably masking a bigger problem but it was a cheap stop gap measure for now while I keep trying to track down the actual culprit.

Which scan tool did you buy? I'm wary of the cheap Bluetooth ones they sell on Amazon but if they actually work I'll get one in a heartbeat.

Taking the old plugs out they looked like they were running relatively hot but it couldn't be that accurate since I didn't check them right after it was pinging like you said.

I haven't checked the fuel pump pressure but I should just to eliminate it (need to get a gauge). I replaced the pump with a Carter unit in early 2015 to fix a bad sender so I kinda doubt it's failing or something is plugged but anythings possible.

I've also noticed my overhead console is reporting a drop in the average MPG by about 1... from 21ish to 19.8-20.