Tsaani97xj

Have a 97 SE 4.0 AX15 and for years on 87 octane it spark knocks/detonates, whatever it is officially called, under even light load. It is much worse when the AC is on. When running 93 the symptom is severely diminished but not completely gone. Octane boost has a similar effect. Forgot to mention the motor is completely stock with 264k. A few years ago I tried the normal tuneup stuff to try and remedy it with no success. I've been lurking here for a while and decided to look for some advice. It used to be my wife's DD and she didn't complain about it so out of sight out of mind. I gave up my mustang and took it over last fall when she bought her KK.

I'd appreciate if anyone had a suggestion on possible causes. I am thinking it's obviously timing related but since adjusting the distributor doesn't advance or retard the timing I don't know where to start.

Tsaani

cruiser54

Run a can of BG 44K through in one full tank of gas. Could have hot carbon deposits lighting the fuel off prematurely.

Radi

I'd bet on it. Might pull a couple plugs too and see what they look like.

We used to fix that years ago when it was a real common problem with a coke bottle full of water, slowly dribbled into the carb (throttle body these days). Just enough to almost stall the engine when you hold it at 1-1.5K rpm or so. That steam cleans the hell out of the cylinders.

Sometimes an Italian tune-up (I'm Sicilian so I get to call it that, lol) works too. Lock the trans in 1'st gear, floor it to just short of redline, then take your foot off and coast back to a crawl. Repeat. Repeat. Repeat.
That heats it up good then alternately creates high engine vacuum to eject the crap that built up. That's a SOP procedure with grandma-driven Northstars and it works.

cruiser54

I'm familiar with the Alfa tune-up. Restored a 79 Spider.

Remember this?

Pretty easy.
Get a cup of water, preferably something like a Dixie cup.
With the air cleaner off, hold the engine RPM at 2000 RPM or higher and dribble the water from the Dixie cup, which you've pinched to look like a spout on top, into the carb or throttle body. Go slowly and the engine will bog a bit. Stop and let it recover. Repeat until the cup is empty.

882door

Seafoam.

cruiser54

44K will do a much better job.

882door

Agreed. Just not readily available.

cruiser54

Ebay. High end car repair shops.

Tsaani97xj

Thanks for the input so far. I think I tried a can of Seafoam when I did the tune up but not 100% sure. It could have went into the mustang that burned 2 quarts of oil almost every tank and fouled the no 8 plug every couple/ few weeks. Ive been interested in seeing what that bg44k stuff was about. It is definitely an easy first step. I wouldn't have considered carbon deposits pre igniting the fuel but that makes a lot of sense.

That leads me to another thought. The O2 sensors are original and the downstream connector broke during a clutch repair last december. This makes it run rich Ive read. Wouldn't that lead to excessive carbon? Seems like it would if I understand how this works correctly.

Again, I appreciate the insight. Some things you just don't consider until they are right in front if you.

Really enjoy this community. Good people on here.

Thanks

882door

Original? Oh God.. Change those right now. When they go bad you'll run rich.

Tsaani97xj

I am bit ashamed I admit it. Up until the connector broke I never knew that it was a regular maintenance part. No check engine everything must be ok. Live and learn. Just always seems like there is something more pressing to spend the 70$ on since it runs the same as it always has.

Just for the record this XJ has been in the family since new. Brother-in-law to sister to dad back to sister to wife to me. I blame them. Jk of course.

I will move them up the list of needs/wants.

882door

Honestly nobody really knows.

It's just one of those things that nobody tells you, and if your at a shop they try to ream you on the price so you naturally you think they are trying to screw you and you won't say yes.

I would see if you can borrow 02 sockets from the parts store. Most will loan tools. It's gonna make it worlds easier.

Radi

Don't feel bad. I'm still running the front OEM sensor in my '96, nearly 240K miles. No CEL, the gas mileage hasn't changed, the numbers are fine on a scan tool. I'm not going to change parts until they actually go bad! I'm sorta stubborn like that.
Are they shot? I don't know- you tell me. All indications say they are not.
I ran the OEM sensors in DW's chevy for 260K. Same thing, still worked fine up until the day the car was totaled out.
I dump a can of Techron in the tank 2X a year and always have, that may or may not have kept them clean.
Besides, they probably won't come out now without torching them out, lol.

Anyway, if you have access to a scan tool that will read real time short term fuel trim numbers, take a look. The front sensor should quickly switch from a small positive indication to a small negative, back and forth without any large spikes and without sitting stuck or acting sluggish.
The rear, post-cat sensor should sit almost unmoving just slighty positive or negative. (If it mimcs the reading of the front sensor, your cat isn't working)

If that isn't what you are seeing, either you have a problem that's causing the engine to run too rich or lean- like a vacuum leak, stuck injector, bad FPR, etc- or the o2 sensor is bad.
If you are seeing consistently positive numbers, the O2 sensor is telling the PCM to add more fuel. If there is no physical reason that more fuel is needed (vacuum leak, weak fuel pressure, bad injector etc) the O2 is making you run too rich and you'll get carbon among other things. The trick is determining whether the o2 is lying to you, or it's merely reporting a problem elsewhere. If the numbers never change, it's the sensor.

DFlintstone

Dang Chuck. U been taking night classes or something?

Radi

Heh. I'm a committed insomniac night owl. Finished rebuilding the steering control valve on my new project car a bit ago and just came in to enjoy the A/C for a while. It's still hotter than blazes in the garage and humid. And full of mosquitos.