The car:
1989 Jaguar XJS Convertible 5.3L V12

The issue: The car idles fine. After starting cold, I can open the throttle and increase rpm without issue. Within a couple minutes of running/warming up, it still idles fine, but opening the throttle the engine is reluctant and rpms don't increase. See the linked video here when just started, after a minute, and then after a few minutes.

Video of the issue (TikTok)

Background: Here's how I got here. 7 years ago (2014), the car ran great but the starter was going out and so we parked it until we could fix it. Fast forward to last Fall (2020). I finally got a chance to look at the car. Found that the issue was the starter relay just needed to have contacts cleaned up. That was a quick fix. But by now it had sat a long time, always in a garage. It wouldn't start.

Performed the following work:

• Replaced spark plugs
• Replaced plug wires
• Cleaned cap and rotor
• Clean fuel tank / sump tank (so much sediment in the sump)
• Replaced fuel filter
• Pushed new /clean fuel through the system
• Oil change

After that work, we got it started!
By now it's Winter and time for storage until Spring (2021).

After storage the car is still starting easily and we're optimistic that we'll be driving it this summer. Checking for leaks everything is looking good, take it down the road a half block and turn to return home. Notice a lack of power under acceleration.

Performed the following work:

• Replaced the fuel pump
• Replaced the left side fuel pressure regulator (I understand the right isn't important?)
• Verified resistance of TPS when cold and warm, no jumps or flat spots (at the wires in the engine bay, not at ECU).
• Verified resistance of temperature sensor when cold and warming. (Both at the sensor and at the ECU the trunk).
• ECU seems to have good vacuum (not tested with guage).

None of the latest work has any impact on the issue. It feels like it's starving for fuel. But why not after just starting? Why does it get worse the longer it runs? Any thoughts, ideas, suggestions appreciated!

 

There is a good chance that the tiny metal filter baskets in the injectors have been clogged with debris. As soon as they are asked to flow more, they cannot. Remove the front two injectors and, coil positive removed from contact, ask a helper to turn over the engine on the starter and see what the spray pattern looks like. Unless it is perfect then the injectors need an overhaul. Do a search, lots on here about how to do it. Also check the injector loom for hard insulation, cracks etc. It is always a good plan to renew it.
The ECU is very unlikely to be the trouble. What you have done so far is excellent, so one more push!

 

CNicko,

Welcome to the Forums.Please do an Intro in teh New Members Area when time permits.

Couple of questions:

1) 1989, most are Marelli systems, different markets call the years different to others, and a few are Lucas, as in the Ignition system.The Marelli has 14 HT leads, the Lucas has 13 HT leads. You have changed them, so 13 or 14?, that will give us a clue to assist much better.

You have checked the CTS, good, and its within spec. I suggest CAREFULLY pulling back the rubber boot and checking the wiring inside the plug, as they do fail. Usually these are NO start engine, but checking is free.

Have a look at the rear of each Inlet manifold, there is a bracket, with the throttle horizontal rod inserted in a rubbery bush, these bushes fail, about 5 years, and your symptoms are common to that failure.

Then it becomes Lucas versus Marelli, so we wait for that result.

 

Thanks Greg. I did check that the injectors were giving a good squirt of fuel a while ago. I'll need to revisit that though as the last time they were maintained/rebuilt/replaced was likely a decade ago, before I owned the car.

 

Hi Grant. Thanks for the input. It's the more common Marelli ignition in this '89, 14 leads.

I'll hopefully have some time and good weather to check those bushings and report back.
​​​​​
Craig

 

Just out of interest how much soot can you see in the exhaust - if you have it warmed through and hit the gas any soot - cough / splutter - too rich perhaps.

Could this be the vacuum timer business and a timing related issue ?

Just thoughts

 

TKO 5speed if your into that see below.

I would check your timing.
I know this may sound silly but did you check the carbon button to the rotor.
I say this because I went through the same things and it was getting worse. Finally I saw the carbon button was just about gone.
I went through things several times but just missed that. It is likely to be a simple item that was thought to be good but isn't.
The XJS will get you down during the hunt for the solution but when you find it its like winning a prize.

Here is my 5-Speed story
I just finished my TKO-600 5 speed conversion. It took 7 months with Corona delays. I did the whole thing by myself. I bought a used 1994 rear with a 3.54 diff. I also had my ECU modified from Roger Bywater and got the TT extractor exhaust.
I had doubts and problems along the way but I stuck to it. When it come to the money I closed my eyes. This was a Corona project! My life goals changed a bit. The Tremec and kit was about $5000. The 1994 rear end (32,000mi) was about $2000 . Roger Bywater's Magic and TT extractor pipes were $1800. then misc TPS, IG amp, plugs, wires, distributor coil, fuel pump, fuel filter, replaced injector hoses ,and then was still having problems after $10,000!!! This thing ran ok until it went to close loop (warm at idle) but the power was lacking the car was jerking the shifts were horrible
After all this time and money I was doubting my decision to do this myself. The directions came from different sources which did not help. I am not a Mechanic but I am getting close now. Finally I took a look under the distributor cap and saw the problem. I took the oversized carbon button from my xj6 and sanded it down the the right size for the v12. It worked. The car ran amazing.
I have only taken it out about 7 times. The shifts are perrrrfect. The feel of the car is un-describable and the sound of the TT pipes puts a smile on my face. This car has an organic feel to it, and it it scary fast. I loved my kickdown when I had the TH400. This 5-speed connectedness blows away kickdown. I can shift in 1st and take my foot of the peddles and roll, or hit it and shift at 6500rpm.
Conclusion: I think it was so worth it. I am looking forward to the summer. Good luck

 

ooops. Hit reply by accident...

Check to see that the TPS is reading at about .32 - .36 volts with the key in the ON position but not started. I'm sorry,,, I THINK it's the yellow and red wires. I could be wrong. They have to be back probed... Move the throttle wheel and look for change. Maybe this was mentioned.

Get a bit of fine sand paper and check and clean WELL the ground points at the rad on both sides. Also, ground points in the fire wall and ANY others you see while you're at it. It really helps things.

Also,,, unplug the coils and make sure that none of the male tabs way down inside of the coil receptors haven't bent sideways and out of place missing their female connector points. Sounds silly, and is - ask me how I know...lol. I could tell a story...

As Grant said, the linkage in the intakes. If the butterfly in the throttle body are NOT moving/opening and the throttle wheel on top of the throttle stand IS (sending a signal for fuel to the ECU),,,, what happens ain't pretty. A ton of fuel will dump into the cylinders while no air is there to mix with it. Slop in the linkage sucks. When it's cold the AIR that is needed is being supplied by the AAV on the LH water rail just under the rods from the throttle stand. You'll see it. It's attached to the LH intake by a cute 1inch ID rubber elbow - that will crack and fall apart if ya mess with it, lol...

Also, grab, pinch and wiggle the area of the wire loom that feeds/connects the coils, the ignition amps, and a couple of other things in that area. If youre looking at the engine from the LH fender, you'll see the rail and injectors. It's roughly between the 4th and 5th injector back from the front. This "interesting" connection point and JUMBLE of wires (taped over of course) is the site of a terrible connection of 5 or 6 very important 12v+ wires that feeds the the ignition and fuel injection system. Mostly ignition. It really is a pretty bad thing, when it's bad under there.

Also check the condition of the 2 coil wire plugs themselves. Mine needed to be replaced entirely.

The timing on the Marelli cars is electronic and not a vac advance system...

 

Update:
This issue still plagues me. Thank you all for the feedback.

More detail about the car:
This 1989 has the 14 lead Marelli ignition and ECU controlled timing advance, so none of those mechanical seizure issues here.

Attempts to narrow down the issue:
Coolant Temp Sensor-
Bypassing the CTS with a small jumper wire resulted in a no start situation. Waiting to bypass the CTS until warm resulted in no discernable improvement over the issue I'm chasing.

Vacuum leak-
Several people on various forums have suggested a vacuum leak. And if you listen to the originally posted video, you can hear a whistle around half throttle. It sounds like that is coming from Bank A, around cylinder 4 in death valley area. However, spraying for vacuum leaks in that area didn't give me a noticeable rpm jump. But I did get a jump when some spray got pulled in through the Bank B airbox inlet at the front. There is also a small vacuum leak in the hose connections under the Bank B intake butterfly area. More checking to do on these things.

6 of 12 issue-
Some have suggested making sure that I haven't lost an entire bank of cylinders as a common issue. I have found disparity between banks A and B. While bank B is pulling strong air through the airbox inlet, bank A is pulling almost nothing through that airbox. Maybe actually nothing. No vacuum at the inlet at all. I unplugged the smog pump to eliminate that air source to be sure, but still that airbox is seeing no airflow through it. It's like bank A is only pulling air through the crossover from bank B.

I then hooked up the inductive timing light and clipped on leads from both banks. I'm getting spark to both banks. Any lead I clip to shows me at 860 rpm at idle and each coil lead independently at 5K+.

Watching the exhaust at start up, bank B side is clear, while bank A has white vapor for about a minute. Putting my hand behind each, bank A feels 'smoother', while bank B pulses more and has more oomph. Bank B exhaust feels to have maybe twice the flow of A. Both are similarly warm.

Distributor:
Because the whistling vacuum sound appeared to be coming from the area near the distributor, I took that opportunity to take the cap off and check things again. Cap, rotor, and connections all look great. This distributor has two hoses connected the the base. Why? One connects the the area under the B intake butterfly as mentioned earlier, the other routes the the left side fender and terminates with what looks like a filter of some sort with an open end that could be connected to another hose, but it's not. These hoses just go the the open space under the distributor cap. Seems odd.

What's next:
Any more ideas? I clearly have a vacuum leak somewhere. But significant enough to cause that major of a stumble to where it simply cannot pick up rpms? Maybe I need to take the exhaust down to see if I have a plugged cat, resonator, muffler issue?

Thanks in advance.

 

Ok.

I just watched that video and that IS strange. Imma think on it. The difference between cold and hot,,, when you turn that throttle is crazy!

If folks haven't,,,, I suggest watching that short clip...

Where IS your FInjection loom... Down in the V?

 

Check O2 sensors
make sure you are not getting oxygen in your exhaust system
if so the O2 sensor would flood you with too much gas

Just checking the TPS may not mean you have a good one.
I actually spliced in test wires to the red and yellow wires.
you want to get .32v to .36v and be able to reproduce the reading in the .32-.36 range
I bought a new TPS but would see some inconsistences here or there.
I then bought a used one and it was night and day. the used one worked as it should.
I convinced myself that it was good cause it seemed right but it wasn't.

Timing would be the next thing I would look at.
My B-Bank is stronger than A-bank also but it should be close. Some of it is because design.

I did all the things you did and had similar problems when it warmed up.
The vacuum lines to the distributer were for ventilation and the silly filter at the end

Butterfly's have to be correct.
I did change the fuel injector hoses.
You will find it!!
I opened my ignition amp and removed the Cap and disconnected the zener so nothing could short out. If you do that don't have a plug wire off!!!!
What can I say keep at it. You need fuel at the right proportion. and the electric to blow it up.
Check wires connections ground points ect.

Good Luck

 

Hi CNicko

Attempts to narrow down the issue:
Coolant Temp Sensor-
Bypassing the CTS with a small jumper wire resulted in a no start situation. Waiting to bypass the CTS until warm resulted in no discernable improvement over the issue I'm chasing.

Putting a Jumper Wire across the CTS Plug/Socket, shouldn't result in a 'No Start Situation' as it should either make the 'Car Run Better' or make 'No Difference At All' which could point to a problem with either the Plug or the Loom Wires going into the Plug or a short to Earth in that part of the Loom

The Harness to the CTS Plug could be shorting to ground, as that sounds quite a likely possibility to me

Because:

While the CTS may have been providing a Slight Leakage to Earth when the Engine was Cold as the Engine Started Warming Up that Leakage to Earth may have increased

Then when you 'Jumpered' the CTS Plug, the Engine wouldn't start, making a short in the CTS Loom look increasingly likely

 

Update: Still unresolved.
It's been a very busy summer and I have had little time to investigate and try suggested remedies, but wanted to pop in with an update.

Oxygen sensors: Replaced
This was a simple procedure and I had no idea how old the sensors I replaced were, so probably needed regardless. I used Bosch O2 sensors. There was no change.

TPS: Replaced
The original Throttle Position Sensor pigtail had wires with cracked insulation and exposed copper. I felt I needed to replace this, even though it was testing fine. I adjusted the idle position of the new TPS to .34V and verified smooth operation throughout the range reading 4.2V at WOT. I hoped there would be some magic, but no change.

Next to try: Vacuum, AAV, CTS wiring
Still hearing a whistle sound, but seems like a minor issue. Nevertheless, large or small, vacuum leaks need to be addressed. Thinking I'll pull the throttle bodies and make sure no air is getting pulled after the butterflies.
It idles fine, so I don't think the issue is with Auxiliary Air Valve, but want to investigate this component as well because I understand these sometime cause problems.
It was suggested there may be a short in the loom wiring around the Coolant Temperature Sensor. I haven't investigated this yet, but I want to trace this out and verify.

 

After reviewing all these things I'm surprised it hasn't been accidentally fixed in the hailstorm of repairs youv done. Seemed similar to my car but much worse in your situation in any case I found having someone holding the throttle partly on while I move adjust and wiggle various wires has helped to find issues. Suddenly Problem X goes away when I moved Wire A! aha! got you!

I still find occasional issues with the marelli loom plugs at the ign coils they are very small brittle wires that can break/move inside the plastic - this being a warm/cold thing I'd guess a partly broken wire somewhere is losing contact with thermal expansion in your case.

My other thought is that maybe the engine heat is causing a fuel line to partly collapse and starve the engine? Unlikely but if those feed/return lines are as old as time then it should be on the list anyway.

 

Did you do a compression test on the engine?
If the cooling system doesn't get hot enough the CTS will play with your fuel. Use 90 degree thermostats with jiggle pins.
Good Luck