XJS ( X27 ) 1975 - 1996 3.6 4.0 5.3 6.0

Engine splutter and no power beyond 3000 rpm

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Old 12-24-2021, 08:02 AM
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Default Engine splutter and no power beyond 3000 rpm

Hello all you learned members. I’m just seeking some support and clues as to what might be a problem I’m experiencing with my recently acquired 1991 XJS 5.3L convertible with Marelli ignition.

Problem:
- When the engine is warmed up, it will not free rev nor accelerate under load beyond 3000rpm. At which point it feels quite asphyxiated and splutters. When cold, it is perfectly fine.

What have I done and checked to date?
- new set of NGK plugs and gapped correctly.
- new distributor cap and rotor button
- pesky coax cable to ECU checked and fixed
- fuel tank removed cleaned and replaced
- new fuel filter
- checked and adjusted TPS settings
- AAV operation checked and confirmed
- Vacuum to ECU checked
- fuel pump connectors checked and voltage at terminals confirmed to be 13.2V

So what might be the source of my problem? Is it perhaps one of the 136 sensors on the engine that decides that it is far to warm and tiring to continue working? Or simply one of the time bomb gremlins the Jaguar engineers decided to hide in one of the wiring looms for a bit of fun?
Appreciate any advice that anyone may have, based on resolving a similar experience.

Cheers;



 
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Old 12-24-2021, 10:11 AM
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Pull both oxygen sensors and see if it revs better. May be a plugged/melted catalyst, especially if it had a failure of one side of the Marelli system in the past. It may not rev to full potential, but it would be better with less back pressure with the open O2 sensor holes. Someone with a scope could do an in-cylinder compression check and look for a blocked catalyst.
 
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Old 12-25-2021, 09:02 AM
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I've gotten to be a hammer with this info, thinking many issues might be the result of an problem I had a while back... Hammer - everything looks like a nail, idea... That said...

There is a jumble of wires (4-5 12v connections) that come together in the loom between cylinders 4 and 5 on the LH bank. The wiring of the junction under the tape in the loom gets really crusty and should be checked no matter what, IMHO...


Sorry for the messy image... You can see where that horizontal line stretches - 5 something wires PLUS the feeding 12v white wire.

This is what I found when I pealed the tape back. Redone, all soldered and rejoined, and the issue I was having went away. Same sputtering and strange ignition issues.
 
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Old 12-26-2021, 12:34 AM
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Originally Posted by JayJagJay
There is a jumble of wires (4-5 12v connections) that come together in the loom between cylinders 4 and 5 on the LH bank. The wiring of the junction under the tape in the loom gets really crusty and should be checked no matter what, IMHO....
Thanks for the suggestion. Pulled my loom apart and found a similar mess of wires joined together. Looked like some dry solder joints so I resoldered, just in case. But alas, that did not fix the problem.
It still just won’t rev much beyond 3000rpm under load, as it misses and splutters, a bit. Anything below 3000rpm, it drives very nicely, bit just does not like having the pedal to the metal! Go figure!
Slowly running out of things to check and replace. 😕
 
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Old 12-26-2021, 07:55 AM
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Originally Posted by XJCCAT
Thanks for the suggestion. Pulled my loom apart and found a similar mess of wires joined together. Looked like some dry solder joints so I resoldered, just in case. But alas, that did not fix the problem.
It still just won’t rev much beyond 3000rpm under load, as it misses and splutters, a bit. Anything below 3000rpm, it drives very nicely, bit just does not like having the pedal to the metal! Go figure!
Slowly running out of things to check and replace. 😕
Right on!

Hmm. The exhaust restriction possibility mentioned above is a good one as it sounds like you've checked many usual suspect type things...? I removed all CATs after an exhausting (no pun intended) global search for two catless OEM downpipes. I still have a little work to do there to get things just so BUT,,, I swear it made a difference - even though many say that it may not...

Simple suggestion... Have you run a bunch of Techron fuel system cleaner thru the system? Anything like that? At lower speeds restricted fuel injectors and a valve(s) that can't properly seat due to specks of carbon my not be a big deal, but at higher revs,,, different.

What's the condition of the distributor and rotor?
The coils?
These things made a difference for me as well.

It's amazing to me how a 700lb hunk of engine can be so sensitive and dramatically changed because of a piece of 9oz plastic like a distributor or/and rotor - or a loose or crusty wire. Never ceases to amaze me.
 
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Old 12-26-2021, 07:56 AM
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Plug gaps at the needed .025 (I think is the number)?
All of them correct? Even the often neglected few?
 
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Old 12-26-2021, 09:00 AM
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Not revving above 3000 rpm is often one of the symptoms of not reattaching the second coil on Lucas ignition cars. They'll run fine on one coil at low speed but need both at high speed. But you don't have a Lucas car. This makes me wonder about your coils though. Have they been checked or replaced?
 
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Old 12-26-2021, 03:57 PM
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Originally Posted by garethashenden
Not revving above 3000 rpm is often one of the symptoms of not reattaching the second coil on Lucas ignition cars. They'll run fine on one coil at low speed but need both at high speed. But you don't have a Lucas car. This makes me wonder about your coils though. Have they been checked or replaced?
Yep, replaced the 2 coils yesterday. Thought, what do I have to lose, other than another $120, down the pit! I was right! Lost $120, which made no difference.

Oddly, what I haven’t replaced yet were all the spark leads. Only because I checked the resistance on these and they were all under 8KOhm. Might they be breaking down when warm and only at high revs? If so, I should probably order a new set. That will round out all the ignition system having been replaced.
 
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Old 12-26-2021, 05:12 PM
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Try pulling the O2 sensors first and see if that helps. If so, you have at least one side of the exhaust with a restriction.
 
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Old 12-26-2021, 05:26 PM
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Coils new,,, leads under 8 ohm's,,,, hmmm...

just throwing things out there now,,,

dangerous question,,,, are you using an inlet FPR on the RH bank of the fuel rail? And how's the one on the outlet? Any gas in the vac hose(s) or, is it possible the diaphragms innerdz are brittle/stiff/plasticized? Honestly, I don't know what makes up the innerdz of the FPRs... Is it a rubber or plastic diaphragm? Anywho,,,

Have you had the car running in an intentionally dark space looking for electrical storms in the V?
 
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Old 12-27-2021, 12:20 AM
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You must be experiencing either fuel starvation, weak ignition or clogged exhaust.
if I understand it correctly, the engine runs as it is supposed to as long as it’s cold ?
That excludes the clogged exhaust in my opinion. You have basically changed all ignition components, so I would arrange a fuel pressure meter visible from drivers seat as next step and see if pressure drops under load at 3000+ RPM
 
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Old 01-05-2022, 01:57 AM
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Well, it’s been an ongoing chase, but I think I’m getting closer to finding the issue. Really feels like a fuel starvation problem to me, but went the easy route first by replacing all electrics. Had to roll he sleeves up today and jump into the fuel system. What I found wasn’t pretty! Sump tank looked like it was full of coffee grounds and injector baskets looked to be close to 90% blocked. Anyway, they’ve all come off and I’ve sent the injectors away to be tested, cleaned and/or replaced, as may be deemed necessary. Meanwhile I’ve cleaned out the sump tank and strainer, whilst I waite for the refurbished injectors to be returned before I reassemble and try again.
some photos of my findings attached for the record. …… feeling quietly optimistic. 🤞

Injector basket 90% clogged

Brown gunk coming out of sump tank …. Not a good sign.

Sump tank full of coffee grounds …. YUK!!

Sump tank tea strainer, doing its job, but not well enough.

Tea strainer cleaned and ready to go back in.
 
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Old 01-05-2022, 09:11 AM
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Originally Posted by XJCCAT
Well, it’s been an ongoing chase, but I think I’m getting closer to finding the issue. Really feels like a fuel starvation problem to me, but went the easy route first by replacing all electrics. Had to roll he sleeves up today and jump into the fuel system. What I found wasn’t pretty! Sump tank looked like it was full of coffee grounds and injector baskets looked to be close to 90% blocked. Anyway, they’ve all come off and I’ve sent the injectors away to be tested, cleaned and/or replaced, as may be deemed necessary. Meanwhile I’ve cleaned out the sump tank and strainer, whilst I waite for the refurbished injectors to be returned before I reassemble and try again.
some photos of my findings attached for the record. …… feeling quietly optimistic. 🤞

Injector basket 90% clogged

Brown gunk coming out of sump tank …. Not a good sign.

Sump tank full of coffee grounds …. YUK!!

Sump tank tea strainer, doing its job, but not well enough.

Tea strainer cleaned and ready to go back in.
Great Sleuthing!!!
Good stuff, Man!

Let's see what happens!
 
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Old 02-05-2022, 12:38 AM
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Well, it’s been a little while sorting out a few other items, whilst I was up to my elbows in the engine.
In addition to everything done so far, I’ve also;
a. Removed the fuel injectors and sent them away to be service (found six of the twelve to have poor flow rates and bad spray patterns along with clogged baskets. Had them all ultrasonically cleaned, baskets and pintle caps replaced. All retested and confirmed good.
b. Cleaned out fuel rail
c. Replaced fuel filter again
d. Checked to see if there was any fuel in the vacuum lines to the FPR on both banks and there wasn’t. (Not sure if they could still possibly be at fault)
e. Replaced the dodgey looking EFI harness with a new one.
f. Replaced all spark plugs and regapped again, in case one was faulty. (Had that happen to me once before where one of the new plugs had failed)
g. As a side; AC compressor found to be faulty so tossed out that Detroit iron and installed a new lightweight modern unit

All reassembled today and feeling optimistic! Started and took it for a drive, found it to be running so much smoother and a pleasurable sedate cruiser. This is looking great!
Now put the pedal to the metal, drop back to second and watch the revs steadily rise up to pretty much bang on 3000rpm and SPUTTER, SPIT, GROAN 😫…. Same again, no difference…
Running out of ideas here. Though I have yet to place a fuel pressure gauge on it to see if correct fuel pressure is being maintained, perhaps that is next.
Other random observation is that the rev counter appears to stick on 0 when first starting on occasion. If that is faulty, is it a possibility that the rev counter could be causing the problem? Just seems weird that it consistently occurs around 3000rpm and distinctly worse when engine has warmed up.
Starting to get somewhat disheartened and bank balance is running low.
 

Last edited by XJCCAT; 02-05-2022 at 01:12 AM.
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Old 02-05-2022, 02:01 AM
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I suggest:
You must verify fuel pressure from the driver's seat.
You should if not already done so, replace the ignition amplifiers test again. The spark could well be breaking down at higher loads, also the loom between the amps and the distributor might need rebuilding.
You should also replace the HT leads if you have not done so.
What if anything have you done to the distributor cap and rotor?
I would in any event replace the fuel pressure regulator on B bank (this controls the pressure) and bypass the one on A bank.

The fuel pressure side of the problem is easy to verify, I would settle that first. My belief is that this problem is ignition, but get the fuel side of things definite before doing anything else.

 
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Old 02-05-2022, 04:22 AM
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Hi XJCCAT

Don't get too despondent, there are loads more things you can try before you go throwing anymore Money at her

The 'In Sync' opening of the Throttle Bodies is absolutely CRUCIAL to the Smooth Running of the Engine, as its a bit like trying to Balance a set of Twin Carbs

And so for Starters the Adjusting Rods Need to be EXACTLY THE SAME LENGTH

Did you by any chance unscrew them, instead of just 'Popping them off' where they 'Clip on' ?

These are Not what adjusts the Gap in the Throttle Bodies themselves, which is done by a Separate adjuster, where the Gap between the Blade and the Throttle Body, should be 2000 of an inch or 0.05mm

But don't go messing around with that yet, as it's much more likely that the Length of the Adjusting Rods are out, as 'REPEAT' THEY NEED TO BE EXACTLY THE SAME LENGTH! and just long enough for a little bit of play, as neither one should be pulling on the bit that moves the Blades inside the opening of the Throttle Bodies , which as I said could well be OK as it is

Though having said that I would also clean any build up of Carbon or Gunk, in the Opening of the Throttle Bodies with some Carb Cleaner on a piece of Rag (Though obviously NOT while the Engine is running!) so just take off the Cover of the Air Filters and then once the Filter is also removed, you should have enough room to Clean them without any problem

As any Carbon Build up Can affect the Closing of the Throttle Body Blades and Artificially put that Gap right out

So try that first and see if that makes any difference
 

Last edited by orangeblossom; 02-05-2022 at 04:24 AM.
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Old 02-05-2022, 04:25 AM
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Yes Greg,
New distributor cap and rotor. Not really convinced it is an FPR issue.

“The loom between the Amps and distributor might need rebuilding”?? There is no loom to the distributor. You mean the coils, which wire back to the ECU, as with the amps.
As indicated earlier, it ONLY exhibits the problem once the car has warmed up, so it has to be heat related. Some sensor maybe or ????. Also, once it does occur it will begin to splutter either under load or free reving, once it hits 3000rpm. Just seems to be very precise as to when the problem manifests itself.
 

Last edited by XJCCAT; 02-05-2022 at 05:00 AM.
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Old 02-05-2022, 04:36 AM
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Originally Posted by orangeblossom
The 'In Sync' opening of the Throttle Bodies is absolutely CRUCIAL to the Smooth Running of the Engine, as its a bit like trying to Balance a set of Twin Carbs
And so for Starters the Adjusting Rods Need to be EXACTLY THE SAME LENGTH
Well, this was one of the first things I corrected when I acquired this car, as they too were out. But now they are perfectly the same. Hell of a lot quicker making the necessary unjust entry on this, than it took to get just right and balance all 4 carbs on my E-type, which took a good 1/2 day.

But needless to say, this is definitely not the cause of the problem which only manifests itself when the engine is warmed up.

Cheers;
 
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Old 02-05-2022, 05:05 AM
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Hi XJCCAT

Now it's onto Plan 'B' The Gap on the CPS Crank Position Sensor on the Front of the Engine, which at the very least may need a Clean with some Carb Cleaner

The Gap should be no more than 0.042

Plan 'C' Coolant Temperature Sensor Pull the Plug off and Test the Sensor with your DVI Meter Set on Ohms, you should get a Resistance of about 2 Ohms, if No Resistance then it needs replacing but also Bridge the Socket on the Coolant Temperature Sensor Plug with a Paper Clip or Piece of Wire

As the Coolant Temperature Sensor Triggers the ECU to open the Injectors to ensure the optimum quantity of Fuel to the Injectors

Bridging that Plug will 'Trick' the ECU into opening, as if the Engine has started to warm up

Ready with Plan 'D' if that doesn't work

 
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Old 02-05-2022, 05:07 AM
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Originally Posted by XJCCAT
Yes Greg,
New distributor cap and rotor. Not really convinced it is an FPR issue.

“The loom between the Amps and distributor might need rebuilding”?? There is no loom to the distributor. You mean the coils, which wire back to the ECU, as with the amps.
As indicated earlier, it ONLY exhibits the problem once the car has warmed up, so it has to be heat related. Some sensor maybe or ????. Also, once it does occur it will begin to splutter either under load or free reving, once it hits 3000rpm. Just seems to be very precise as to when the problem manifests itself.
Sorry, to the coils.
Have you renewed the two crank position sensors? These can be heat sensitive.
 


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