I've had my 1999 XJ8 for a little over a year now. From the very first time I took a long trip with it, the car has intermittently had the driveability issue mentioned in the title. Typically it only happens under part throttle acceleration. Usually after the car is warmed up. Most evident when at freeway speeds and you lightly apply throttle to slowly pass someone. Very rarely does it happen at idle.

Otherwise the car runs perfectly and has plenty of power all the way to redline when floored.

There are no CEL codes stored and Performance Restricted has not flashed.



While the problem has been intermittent, it's becoming more frequent now so I figure it's time to diagnose the issue before a hard failure leaves me stranded.

I've done a few things to try and diagnose the issue to no avail. Since it's really bugging me lately I've decided to spend the last 3 hours searching this site using the keywords in the title. Here's a compilation of the possible causes and suggestions that I found:

Throttle Position Sensor TPS signal intermittent - Clean Terminals, replace tin plated terminals with Gold Plated version.

Crack or Obstruction in Full and Part Load Breather Hoses - Check and Clean or Replace as needed.

Obstruction or Leaks at Full and Part Load Breather connectors - Check and Clean or Replace as needed.

Cracks or Leaks in Air Induction System. (between MAF and TB) - Check and Repair or Replace as needed.

Spark Plug Gaps at 0.035" - 0.039"

Clean Mass Air Flow Sensor with CRC Cleaner

Bad Mass Air Flow Sensor MAF, "They seldom give codes" (Brutal)

Intake Air Temperature Sensor

Disconnect and Clean all Connectors in CAN (Car Area Network)

Clean and Check Terminals at Accelerator Pedal Position Sensor

Faulty Accelerator Pedal Assembly

Air Filter Restriction

Fuel Filter Restriction

Poor Spark Plug Condition

Faulty Ignition Coil

Faulty Ignition Coil Boot

Ignition short caused by Oil in the Spark Plug holes due to leaking Valve Cover Gaskets.

Compression Test

Throttle Position Sensor TPS Sweep Test

Kinked Fuel Lines (possibly damaged during fuel pump replacement)

XJR Specific - Fuel Line Issue

Check Fuel Pressure

Fuel Pump Failing

Vacuum Line for the Fuel Pressure Regulator has a 90 degree bend and can kink.

Throttle Motor Signals Intermittent - Clean and Check Terminals

Use and advanced OBDII Scan Tool to Get Live Data Readings

Clean Throttle Body of Oil and Varnish build up.

Clean Air Inlet Tube of Oil Due to Blow By

Check for Cracked, Loose, Missing O-Rings and the condition of all Lines in the Crankcase Ventilation System including the Dipstick.

 

What are the "Live Data" readings?

 

I've done quite a few of the things noted still to no avail.

As I have time I'll go back and update the first post with details. I'll also try to rearrange them in a priority from the basics to the complex.

If I've missed (pun) something let me know and I'll add it in.

My intent with this post is not just to diagnose my car but to leave a legacy that others may use as a checklist when diagnosing driveability issues in the future.

Please avoid quoting the first post in any replies as it will be a living document and will definitely change.

Ken

 

Added to the list.

I don't have a scanner that will do that. For now I'm trying to cover all of the basics. Once I get through those, I'll deliver it to the dealer and let them use the advanced diagnostic tools.

The problem is it's intermittent. The way my luck runs, it won't act up while there and I'll get "No problem found" on the work order but still have to pay $119.00 for the diagnosis.

Thanks, Ken

 

Well, I would say at thius point you need to decide if you intend to fix it or take it to somebody who can dianose it. You usually need live, graphed data to find something like this. Or start changing parts, which will exceed the cost of buying the correct tools pretty quickly.

 

Through Amazon or Walmart.coms you can get an Equus 3040 Scan Tool for around $80. It will let you see "Live Data." www.iequus.com Snap-on Tools sells these, so that should give you an idea of the quality.

Removing the electronic parts and getting back to the old days, your problem would be one of two things. First, I would have checked the accelerator pump because this acts like a lack of extra gas to make up for the extra air that is coming in. Fuel injection does the same thing by squirting extra gas. If the computor is not getting the signal to mimick the accelerator pump, then you will have a stumble. Things like the MAF, MAP, and TPS can do this to you. Number two is if the EGR is opening too fast or at the wrong time you will get a stumble if there isn't enough air and gas to make up for the exhaust coming in.

 

Now you are speaking my language. I can almost rebuild any Holley or Carter Carb with my eyes closed. I can even custom tune them for Blowers, Turbos, Nitrous... EFI however can be a PIA with all the sensors and actuators in the system.

Considering I've been through most of the basics I've listed above my next step is to get into a more advanced diagnosis like live data.

Mike, It sounds as if you are a full time tech. If a car came in with an intermittent stumble issue during light acceleration at freeway speed, would you skip all of the other diag and go right to the live data first? Just trying to get some insight as to what the process should really be.

 

I would change the seals in the accelerator pump. You guys crack me up.

 

WHAT SEALS?

I LOVE IT WHEN A LITTLE PERMATEX #2 IN THE "KILLER BLEEDS" OF A HOLLEY 4 BARREL LETS THOSE SECONDARIES OPEN UP LIKE A TRANS KICK-DOWN.

You need the live data to see what is happening. The injectors have to mimick an accelerator pump or you get a stumble. However, you can't look into the venturies to see it happen. The ECU has to get a signal to quickly respond to a big glup of air. The MAP would be too slow, if the engine has one. The MAF can almost handle it, but it may be a little late. In the past, the TPS told the ECU to give the injectors a bigger shot of fuel. The ECU gets the idea fast.

Now in your case since it is intermittent, I would pull the EGR and make sure it isn't stuck open due to a build up that comes and goes as the carbon sticks and releases. Another member found he had that problem on another make of car. EGR's work the same, Jag doesn't have magic parts.

I was a shop mechanic for over 20 years. We had to fix everything, whether it was an engine that I could hold in one hand or one that needed the shop hoist to handle the heads. Mechanical fuel injection or electronic; we were told to, "fix it!"

IN THE WORDS OF A GREAT AMERICAN WOMAN:
"PARTS IS PARTS!"

PS Look into the throttle body and see what's there. If you feel led to clean it, try squirting a little water at a time while the engine is running. It takes the deposits out of an engine and doesn't remove the throttle body coating that lets you do this. Don't move the throttle plate on engines that don't work the throttle by a cable. You will screw up the throttle stepper motor.

 

You can pay them $119.00 to find nothing or $80 to watch for it yourself.

My Equus 3130 ($130 from above sources or $311 list)can take a series of data "pictures" that can be "played" back on the computer.

 

I had a similar problem and searched for a few weeks trying to find the culprit. Believe it or not, it ended up being the spark plug boots themselves. Over time they tend to loose their "grip" on the plug and can cause the spark to arc. Installed new boots with dielectric grease and the problem was gone.

 

Try that while you're at it. Did they leave carbon tracks?

 

The plugs with the loose boots did have carbon tracking along the ceramic shaft.

 

Years ago on the TV show "60 Minutes," they had a piece about the clean-up of a Super Fund site in the Pacific Northwest. They filmed a library that consisted of a vast amount of research done on the site and what the clean-up would entail. The bottom line of the story was that even though a huge amount of paper had been consumed in all the writings; they still would have to start digging to find our just what was there and what needed to be done.

If you can rebuild a carb then you can do all of this and us giving you theories can only go so far and then, LOL, you will come back with, "Guys, you'll never guess what it was!"

The Scan Tool just does all the stuff that we did with Dwell Meters, Timing Lights, Pressure Gauges, eyeballs, etc.

"GET R DONE"

 

There is no EGR to pull on this engine. EGR magic is handled by the VVT's changing the cam timing and as a result of the overlap exhaust is allowed to mix with the incoming air fuel mix.

No MAP sensor either.

The bright spot is at least there are two less things that could fail.

 

Back in them olden days, you could remove the rotor from the GM high energy distributors and there would be dark "spider webs" on the bottom from the zap leaking. You could also see it on the ignition coil. And who says too much is never enough!

The guys used to come to me wanting me to connect to the ECM and see what was going on. I would always suggest that they see if it needed a tune-up. Usually they didn't need to come back.

 

don't laugh at this...replace your o-rings on the vacuum lines to/from the manifold and throttle body. My oil dipstick o-ring broke in two when I tried to move it...it wasn't even fitting snug into its line anymore... gameblasters 2002 xk8 didn't even have an o-ring on his dipstick. My TB vacuum line (not sure if your 4.0 has one also) can rotate with the slightest of force. I picked up a couple packages of the $3 variety pack at the auto parts store and just fit/replaced with new ones on the fittings until I created a snug connection.

 

Plain old rubber vacuum lines were so much easier

 

The dipstick has on O-ring

I'll add that to the list of things to check.

 

I've ordered the Equus 3040 like Mike suggested. I assume it will be here next week. Hopefully "Live Data" will tell me something.

Otherwise it looks like I'll be parking the car in the garage for a couple of days and going over the checklist.