Video example

I'll link to a video car is in drive, foot on brake. Giving it gas unexpectedly or abruptly seems to be causing near stall conditions. The video is an exaggeration as you'll notice but while stopped behind a car yesterday I gassed it to whip around and it nearly stalled...this is in a 3spd automatic.

Throttle and/or engine responsiveness in THESE conditions feels super sluggish it catches itself most times but at idle speed like backing into a parking stall it nearly dies. Getting alot of boat gas smell to it may be running to rich butI have no A/F meter gauge to know for sure.


Any ideas?

All ignition, vac, injector, etc systems are new to within 40,000km except 02 sensors which are probably original..yikes. Mind you the car absolutely roars to highway speeds without hesitation, downshifts per vac module like a lightswitch, no issues at speed.

 

At first blush it appears to be fuel starvation but, since you say it runs well at speed, perhaps not.....at least not in the sense of fuel delivery to the rail. Perhaps an anomaly with the pressure regulators? A fuel pressure test might be in your future.

A million years ago my XJS suffered major crapping-out and bucking...almost violent at at times. Long story short, the inlet regulator was faulty.

My thoughts turn next to the TPS, which functions to increase fueling as the throttle opens.....somewhat akin to the accelerator pump in many old carburetors which gives a squirt of fuel as the throttle opens. Check the TPS voltage; you may have a dead spot.


Cheers
DD

 

Thanks, that TPS thought occured to me though it was inspected within the last year, along with fine tuning all throttle linkage with new bits...which I guess doesn't mean anything...a peice of dust or general fatigue could of kicked in by now and caused issue.

 

When you did the ignition, did you replace the module if you have Lucas? I think 89 is a change over year, so I don't know if you have Lucas or Marelli.

 

Hi, V!

I experienced pretty violent oscillations of RPM a few months ago, with the car almost stalling (or on couple of occasions actually stalling.) This behavior was typically observed at idle, not when driving at normal/good speed.

Later I heard cracking noise coming from the trunk (which I reported about). I thought that my fuel pump was dying (or starving.)

I recently replaced everything fuel line related in the trunk: new fuel pump, new fuel filter, new sump tank filter, cleaned everything, etc.

Have not experienced these near stalling conditions since I finished the trunk fuel line updates (knock on wood!)

Not sure if this matches your symptoms, but wanted to share...

Good luck! Hope you find what’s wrong with her and fix it!

 

My car is Marelli everything down to the amps was redone, fuel pump crackling is present though Lucas injector lube softens it, but if the pump was an issue I'd imagine the car wouldn't be so fast at WOT?

 

Right.

Generally, if you have a fuel supply problem you'd be more likely to feel it when the engine is under load.....and less likely to feel it at idle or in low-load conditions.

I say 'generally' because, over the years, I've learned that failure modes and symptoms are not always carved in granite. (My XJR, which I called "The Tormentor", was particularly instrumental in teaching me those lessons )

Cheers
DD

 

Tormentor, I'll add that to my list of names for the next one I buy lol, I call my car the Widow Maker given it's predilection for having one front brake caliper not work at highspeed / hard emergency braking. Particularly terrifying when coming down over a bridge several hundred feet above sea at 75mph and you swurve toward the railings.

 

TPS versus linkage lag is not set 100%..

Most, well ALL I have sorted, have had:

1) TPS incorrect split voltage, and near enough is a WOFTAM. Also the spacer behind the idle stop of the capstan 3MM from memory) when setting the rods is 100% a required item.

2) The GOO inside the throttle body can, sometimes cause this lag.

3) Not sure of the Marelli beasts, but it may have a "full fuel vac switch" tucked down near 5A spark plug, and they are designed to richen the fuel a TAD when doing exactly what you did, to STOP that happening.

4) Fine tuning the ECU Idle Fuel Pot is also an item that needs some time spent, and can cause, to a small degree, what you have.

This is the Fuel Valve mentioned, AKA, Blue/White valve. They fail. They are also adjustable, so if your car has one, I will let you in on the secret HAHA.

 

Yes !

I knew I was forgetting something.

I've experienced this problem.



Grant, I know you've explained the logic of this before, and I understood it at the time....but now I've lost it. Can you give it another go, please?


FWIW.....Mine switches at 4" of vacuum. I dunno if that's right or wrong; I don't think I've ever seen a spec. And I'd like to know the secret. Is there a tiny screw inside, as one often sees on transmission vacuum modulators?

Cheers
DD

 

Blue capsule thing is present, I've double checked the linkages they respond to the slightest touch of the pedal, not seeing any slack there.

 

You want a little slack there. 2-3mm rotation before the throttles open.

A couple reasons for this. Main reason is to ensure the throttle rods/capstan allow the throttles to close completely onto their stops. It gets hot under there too. Steel Steel rods expand along their length primarily. This extra room also allows for expansion without cracking the throttles open.

Remove rods.
Set throttle stops so throttles have .002 inch gap at the top.
Put a 2-3 mm thick nut between the throttle turntable and its stop.
Replace one rod. Adjust to just touch but not move the throttle.
Repeat with the other rod.
Remove the nut.

 

THERE'S YOUR PROBLEM.

JJ has answered that part, but I will post the PDF later.

I will respond to ALL this later tonight MY time, as wifey is springing for lunch, and I will then tolerate a few hours off "retail therapy".

 

Thanks, will look again when it stops raining, though when I say no slack I'm not measuring with a pinpoint gauge..in my line of work 1/8th variation is par for the course, 2mil is what we call pedantic. If anything th cr is more twitchy in a good way at hot eng temps. All throttle assy re-done by a Jag racing specialist it *should be flawless...waiting on the Wizard of Oz to tell us his blue capsule secret before doing anything more lol.

 

I had very similar symptoms, and I think the slightly rich smell is the most important clue. An ECU controlled car shouldn't ever smell rich unless something is amiss.

If TPS, linkage, etc. etc. are all good, and you still have the issue, double check your connections of the HT leads into the distributor.

After doing lots of various work in and around the valley of death, I had one of the leads either get knocked or work it's way slightly loose from the cap. Car runs fine in most circumstances on 11 of 12 cylinders except under the circumstances you describe. Drove me nuts trying to pinpoint the problem, and I adjusted everything twice before I noticed one of the leads slightly off compared to the other 11. Pulled it and re-seated it -- problem gone.

Possibly a long shot, but your description was like deja vu so I thought I'd pass along my experience.

 

[QUOTE}

Grant, I know you've explained the logic of this before, and I understood it at the time....but now I've lost it. Can you give it another go, please?

FWIW.....Mine switches at 4" of vacuum. I dunno if that's right or wrong; I don't think I've ever seen a spec. And I'd like to know the secret. Is there a tiny screw inside, as one often sees on transmission vacuum modulators?

Cheers
DD[/QUOTE]

Doug,

That spacer is to allow the TPS to send a signal TO the ECU, a millisecond before the throttles open, and that signal is generally enough to give a slightly longer pulse to the Injectors before the sudden rush of air and drop in Vac.

Each engine is different in this area, and I mostly use a 3mm spacer, as I set the rods, and take a loooong time to set those rods, one side, other side, back and forth until I am happy. Some people laugh, so be it, but none of mine have any lag.

The Blue/White I am doing. I had it, cyber space has stolen it, so i will do it again. Not be long I hope?

 

Here is the PDF "secret" for the Full Load Fuel Valve, AKA, the Blue/White valve.

 

On a friends car (Lucas) it started bogging down on takeoff. it turned out, for reasons we never found out, that the timing had retarded itself by quite a bit, so worth checking that too.

 

Thanks Greg.

Too long on the Batphone with you last night, I forgot the other PDF's.

Here you go guys.

 

Thanks, Grant

For those wondering, the KPa readings on Grant's gauge convert to 5.6 inches of mercury (switch activation) and 17.4 (cruise)

Cheers
DD