New to the forum... 96 Jaguar XJ6 starts fine, idles at 800 in neutral 700 in gear, at times it gets a bit rough drops to ~500rpm and stalls, restarts right away with no issues. Will not rev over 2700 rpm in neutral it just bogs. In drive it accelerates with light throttle but stumbles and back fires with more throttle. Stalled once after a 50 mile trip. I had it at 50mph and went to full throttle and it popped and shuttered and stalled, it appeared to have smoke and raw fuel out the tail pipes, then it restarted fine and drove normal with light throttle. It also stalls from time to time on tip in after stopping. the rpms drop to 500 when it's heading for a stall. I have had codes for miss fire on cyl X (random), trans did not receive engine speed signal. So far I have installed a new battery (it needed one), removed and cleaned throttle body, removed cleaned and tested the idle air valve, cleaned and tested the TPS (via resistance not voltage) removed and inspected the spark plugs, cleaned injectors (shop did the professional procedure), replaced the fuel filter, checked for vacuum leaks, cleaned the crank position sensor and inspected (not much to see), checked the coils at night for light show, new air cleaner, cleaned the EGR valve, then blocked it off for testing, with no change. Considering new fuel pump, new crank position sensor, new throttle position sensor. car has 140K on it and I'm not going to give up, but sure could use a bit of help.

 

If you get a light show you know the coil is bad but lack of a light show doesn't prove a coil is good. Unfortunately.

Anyhooo.....

Coils are a problem as we know but, unless several are failing at once, I'd think you'd get cylinder-specific misffire codes. Since your getting a code for a random missfire I'd be inclined to try a crank sensor if I had to take a stab at something.

Other will have more suggestions.

Cheers
DD

 

Welcome!

Sounds like you know a bit about your car and what to check in this situation. My advice is to replace those spark plugs with new ones. Get the Champion RC12YC plugs. That will cost about 12 dollars. Otherwise do not replace any component unless you are sure it is bad! See stalling post just below.

Consider replacing the Ckps if it has never been done. However, it is cheap and easy to replace those plugs and rule them out as a cause.

 

I agree with the guys above although I use NGK Iridium in mine, one grade cooler as ocasional track use. With my previous x300 I found it was important to buy plugs with the solid terminal rather than the screw on type as the contact on the coil is a thin spring loaded rod which presses on the top rather than gripping the sides like most leads.

 

thanks for getting back to me so quickly. I will replace the plugs first and the ckps second. I am also going to check the fuel pressure. It would be nice if they had put a shrader valve on the fuel rail... is cutting the fuel line the only way to put a gauge in the system. I have had two older cars that had similar problems and had enough rust in the tank that it would plug the pick up screen, so dumb question is the tank metal or plastic? I don't remember seeing it when i changed the filter. I wasn't looking and the thought just came into my feeble mind.

 

The tank is steel. The rev limiter is what keeps you from over 2700- 3000 in neutral.

 

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Thanks for the help, here is an update. New plugs, no change. I have the ckps on order, will have it tomorrow. here is a bit more information. when driving at normal 30-35mph when it stumbles the check engine flashes and if it catches and doesn't stall the light goes out. if not obviously all the lights come on. no codes. if I push it up in the revs when it stumbles the trans light comes on and stays on. I would have thought it would have set the trans no signal code, but no. with the trans light on it seems to run on limp in mode (less power) but it will still stumble when in limp mode.
how would you recommend installing a fuel pressure gauge? I think I would like to have it in the cabin for testing (to see if pressure is the problem when it acts up). High pressure line is not a problem I have lots of it. but the factory lines look like they are shrink fit to the hard lines and I don't know it a standard barb fitting will work on the factory line, hose clamps etc. Thanks in advance.

 

Yeah, the x-300 is a pain to hook up a pressure tester. I thought of drilling the rail and installing a valve but never got to it. I have a long hose on my test guage, so I can tape the guage to the outside of the windscreen whilst driving the car... it seems safer than bringing the guage into the passenger compartment.

I would bet more than a cup of coffee that the tranny light will prove to be the code that means the transmisison and engine speeds disagree. That further points to a likely bad CKPS.

 

So here's the news for today. I installed the new CKPS and it helped with the higher RPM stumbling but it still runs rough at idle sometimes and it still stalls. I think I have created another problem. after cleaning up the front of the engine with some brake cleaner and installing the CKPS when tested the engine wouldn't make any power it runs smooth but takes forever to get to 50MPH I swapped back the old ckps but it still didn't rev and the higher RPM stumble was back. So the new ckps helped. After all that I tried unplugging a number of things one at a time. the IAC- engine continued to run the same and reved the same. the MAF-the engine continued to run but needed slower throttle input to rev up. the timing adjustment device on the front right of the engine (if someone could tell me what this is that would be great) unplugged and the engine continued to run without issue.
Then I thought it could be a fuel pressure problem, so I puled the fuel rail, drilled and tapped it for an 1/8 pipe and installed a shrader valve. I installed my pressure gauge and cycled the ignition check for leaks and then started it. the static pressure was 30psi, and 35psi on throttle tip in. watched it till it stalled and the pressure never fluctuated. So the only thing I have left are the IAC and the TPS one for the idle stalled and one for the "no power"?? Also of note, during a full throttle test the trans down shifted to 2nd and I the revs went up at that moment I heard some detonation. and lifted as fast as I could. I guess the knock sensor isn't working.

 

If you have a scanner that will trend chart the LTFT and STFT at idle, midrange and under power, the results could very well give some clue.

Also, if the CKPS really helped some, you might want to chack the connectors- both at he sensor, but more likely at the ecu itself. Several folks have reported corrosion due to water getting into the kick panel area.

 

I don't have a good scanner. Can I ask what LTFT and STFT are? so do these cars have a tendency to clog up the cats? I'm wondering if all of the throttle body cleaner has caused this. 140k miles. also think I will pull the TB and double check the TPS and the IAC. Can I push and pull on the plunger on the IAC or what tests can I run on it. I'll probably pull the O2 sensor or do the vacuum test to see if the cats are plugged. any other ideas would be most appreciated. Does anyone know what the sensor in the right from of the engine it looks a bit like a distributor (with no plug wires) and is adjustment like that.

 

The distributor looking thing is believe it or not, the cam position sensor. It is simply a pulse switch to time the cam to the crank. The car will start and run without it, since the ecu is smart enough to try both orientations of the cam while trying to start if the CPS signal is not there, and then disregards the signal and just maintains time from the CKPS. Longer crank times can indicate a bad CPS.

LTFT and STFT are the Long and Short TERM FUEL TRIMS. They indicate what the fuel system "thinks" needs to be done to correct the mixture and they can be good indicators of sensor problems, vacuum leaks, fuel issues and so on. A scanner can also tell you if you are going into closed loop control or not.

It was not clear if the engine continued to run the same (bad) with the MAF removed, or whether it continued to run instead of stalling. If it ran better, (Or not!) you should probably clean the sensor element in the MAF. If you do not know the usual cautions on this, look to the archives.

I do not know if X-300s have any tendency to clog the catrs, but it has been reported occasionally. Be careful with powering the IACS while not installed- The plunger, spring, pintle and other parts will eject into earth orbit. At least I couldn't find mine afterward! I don't know how you are going to check the TPS by removing it, but you sure might check with a meter to see if the wiper transitions smoothly between about .5 to 4.5 volts.

Good luck,and keep us posted!

 

I would definitely check / clean the ECU connections as suggested above. I had all sorts of problems a year or so ago which turned out to be that. Use electrical contact cleaner though not WD40 ect. as believe it or not the oil film will cause problems. Mine had a bracket securing the plugs for some reason so I had to drill out two security screws. Be careful when scraping corrosion off the pins!

 

Thanks guys, I'm going to have a look at the ECU plug right now. I have cleaned the MAF and I did check the TPS but did it with resistance not voltage that why I'm going to have another look. BTW when I cleaned the IAC I pulled and pushed the plunger, was that a bad thing?

 

Not if it did not come out in your hand! And always push it in before installation so you do not buger the pintle or seat.

BTW, the way that thing works is that the ecu always drives it full closed (extended) at power down. It bottoms out, then the stepper function electically slips. It then is opened the previously determined count of pulses to the "correct" idle position and adjustments are made from that point to set the idle speed, assuming the throttle is at idle position.

 

So the news for today is as follows. I ran a vacuum test on the intake checking for clogged cats found no problems. On my test drive I got a misfire on cylinder 4 so I swapped the coil and plug to #1 couldn’t get it to kick the code again. I pulled the ECU and checked for corrosion none, all the pins were straight and the connector in the car were good, so back together and test drive, no change. I pulled the air box and MAF cleaned it while I was there and checked the TPS the voltage was correct and smooth. Re-checked everything as it was going back together, no leaks. And the IAC valve moves freely and it never came all the way apart. I’m getting a little thin on ideas. Now wondering if the injectors could be at fault but I would think that would set an o2 code. Or I’m wondering about the coils. Is there a test that I can do? Probably the oldest test in the book but I haven’t pulled the plug out and grounded it to the engine to see the plug spark… don’t know if that is doable / safe with the spring contact in the coil boot?
The car still starts up every time and will stumble and stall from idle or tip in. Most of the time it idles fine, though I noticed when I was watching the engine closely that it cycles from normal to very slight roughness all the time on 6-7 sec. cycles. will not make any power but revs smoothly with the new CKPS. It makes lots of induction noise like those silly “cold air intakes, but everything is stock. It takes a very long time to get to 50mph. I’d be surprised if the thing is making 50 HP. Any ideas or help would be greatly appreciated.

 

Get something to read the LTFT and STFT!

 

Hi sparkenzap,
Can you give me some names of scanners that will read these fuel trims. I only have the $49 special from Auto-zone. I may be able to rent or borrow something once I know what I'm looking for. Thanks.

 

FWIW I use an Innova 3130. Nothing fancy but good enough for DIY work. You can Google it.

Many other choices and prices ranges.

Look for something that reads and displays "live data" or "live data stream" as opposed to just being a code reader.

Cheers
DD