Hi all

As some may remember I had an issue with bad throttle response last year. I fixed the throttle axle bushes and things were good. Or I thought so...

https://www.jaguarforums.com/forum/x...sponse-183502/

I have mounted a new Bosch fuel pump lately. Other new(ish) components in ignition/fuel is ignition leads, coil and plugs. Resistor pack connectors cleaned. Occasional use of injector cleaner in fuel.

Car was running really sweet 2 weeks ago on a 50-60 mile run and also 4 weeks ago doing 150 miles. Both runs were in around 15°C, the first partially in rain.

Yesterday it was a bit warmer, around 24°C. Not warm at all except from if you live in Denmark as I do.
Engine started well, idled well and all was fine until after maybe 5 km. Still just warming up. Engine then started hesitating and slight backfiring at light load, maintaining 60-80 kmh. More right foot action resulted in lots of power, so it seems to get fuel and be able to ignite at load. Quick throttle opening results in backfiring.
I turned around to not get stranded too far from home. When the engine was warm I treated it with 2-3 WOT sessions lifting of at around 150 kmh. Full power is available and dark clouds forming in the slipstream indicates engines runs rich at WOT. All good in my perspective.
After this the car ran just as good as it normally does. When I came home after some 60 km I tried to rev in N. It still backfires at quick throttle opening.

Some background:
1989 HE engine from English XJ12.
No O2 sensors or catalysts, so it must run open loop
Lucas ignition
Lucas 16 ECU
No cold start injector
Overrun valves seems to be present. At least the housings and vacuum lines are there...
Returns around 13,5 l/100km consumption as yearly average (around 21 MPG(UK) or 17,5 MPG (US), so it doesn't seem to vaste fuel...

I have the following list of suspects:
Timing advance failing occasionally.
Bad ignition amplifier or related wiring
Bad injector harness
Sticking injectors unable to deliver correctly at part load
Throttle potentiometer failing occasionally. I have checked and found it OK last year...
Fuel tank ventilation failing occasionally.

Let me have your input and ideas to solve this

 

Leo
I would recheck the TPS for gradual and even change over its range. Also, check if the vacuum operated full throttle enrichment switch is working. It should be normally open until vac in the manifold disappears when it closes and this activates the enrichment. Maybe it is always closed? Also, could it be possible that the injector loom is shorting to earth and the injectors are open all the time? If the loom has never been changed, this is a prime candidate.

 

Thanks Greg

TPS checking makes sense. Even changing it on suspicion could be an idea.

The enrichment switch failing would result in continuous over rich mixture as I understand it ? I guess backfiring would be caused by a too lean mixture (if the mixture is cause of the problem) ? The switch for sure works at WOT as I can see black gusts in my mirrors at kick down.

I have now edited the subject line to be as I wanted. It’s hard not to have English as primary language, and I refuse start using the Google translator...

 

 

leo,

I imagine there are different types of backfiring. Sometimes it is difficult to describe and differentiate between them.

I used to have what I thought a mild backfiring at low RPMs. I heard distinct "pops" (but fairly mild, not violent) when throttling. And not 100% of the time. This type is difficult to diagnose. Sometimes you just need to proceed with principle of elimination.

What I eventually discovered was that my vacuum lines were "messed up" by previous owner. Some were connected incorrectly. One was even completely disconnected. This whole thing led me to the vacuum capsule which is attached to the distributor. Vacuum capsule was seized, too. I ordered a new one and replaced the failed one. While in there, I also cleaned and lubricated the dizzy.
Not quite sure exactly which of these fixes finally did it, but after I was done with vacuum lines/vacuum capsule/dizzy work, my hesitation/backfiring was gone! (Knock on wood.)

 

Is this the violent gunshot type backfiring? Curious because my car will do this when coasting down hill at highway speed. If so maybe I can offer some useful information.

 

Thanks for your replies

My backfire is not violent, more puff than bang. Much like what V1rok describes..

What I had yesterday was more or less zero power from just above idle to maybe 20% throttle opening. Some puffing, loads of power if I just open the throttle more.

I must have fuel, ignition and air, but it does for sure not come in correct amounts and/or timing at low throttle opening

My feeling is that I get a very weak mixture, but the ignition advance by vacuum must also be a potential failure source.

Should the advance change a lot from idle to light throttle ? Or could the timing be wrong at idle as well and the engine then just doesn't mind the wrong timing at zero load ??

 

Leo
Have you checked that the throttles are opening (a) in synch, and (b) proportionately with the capstan and the pedal?


Also, can you swap in a known good amplifier to try? And check the dreaded shielded wire for continuity?

 

Also:

You have tested the TPS at idle and WOT, but is there a SMOOTH steady rise as the TPS is opened slowly. They do get a work out in the 5% to 30% range, and that is where it seems you have the issue.

The ECU fuel "pot" may need a tickle or 2.

Try the "drive timing" method I have suggested to few others, and if you cant find it, here it is.

Trimming the timing on a running V12.pdf

The condensor capsule inside the Ign Amp needs to be removed and trashed, if you have it still installed.

 

Good point with the throttles opening in sync and proportional with the TPS output.
I actually fixed this issue last year after finding one throttle axle rubber bushing completely detoriated. The car ran well afterwards, so I thought I had found the root cause.
What I haven't done this time is to check if sync is still OK. It looks so based on push rod movement, but I better check. That will be on top of my to-do list !

For the amp: I could probably get my hands on a known good amplifier, but I don't have one available here and now.
I have my doubts on this being the issue: How should bad amplifier be able only affect light throttle band ? I would think failure here should influence at all revs ??
Shielded wire has not been touched for years. Maybe worth a shot, but again: Why would it cause light load trouble combined with good idle and good top end ?

 

Thanks Grant

The TPS output rises smoothly, no drop outs or other funny changes.

ECU fuel pot: Well, as long time as it runs fine most of the time, something else must be causing fuel/ignition to get out of correct range.

Regarding timing: I don't have the ignition timing tool needed, but it looks like this is the perfect occasion to acquire one. I generally buy whatever tools I need, so this must be the time for this one...

I (most likely) still have the condensor inside the ignition amp, so I might as well just get rid of it during this round of fail finding. Could it cause the engine to misbehave only at light throttle ?

 

I have noted one more detail: The misfiring/bad throttle response is clearly worst quite shortly after starting the car, and it mostly happens when ambient temperature is above 15°C. It's OK cold, and it's often OK or close to OK when fully warm.

Could it be some temp sensor signal messing things up ??

Leo

 

It could, I suppose. The temp sensor for the ECU is on the B bank thermostat housing/water manifold.


BUT, normally backfiring is a timing matter. Have you had a look inside the dizzy cap for something untoward?

 

I’m taking your descriptions above to mean you don’t actually have a BANG backfire. But a POOF miss or skip. Is that correct?

Have you metered the ATS ( air temp sensor ) and it’s circuit that, on some cars, includes the fuel rail temp switch? The fuel temp switch can fail and opens that circuit. This reads the same as an open ATS or very cold intake air. = +5% rich.

 

The issue I described as backfiring is probably more loads of missing than real backfiring. I get serious hesitation, popping and irregular power delivery at light throttle. It feels like serious starvation or ignition badly out of adjustment.

I can generate proper backfire by aggresive activation of throttle capstan in neutral. I'm pretty sure both my popping while driving and backfiring in neutral is happening at induction side, but I have actually not tried to do it with air filter boxes open. It could be interesting to see if it's happening on both banks.

I have not measured the ATS or fuel temp switch (if I have one...).
My A/C is by the way not working, so I do not have fuel cooling, but I do on the other hand not have really high temperatures here in Denmark.

I have not had the dizzy cap off. Cap, rotor, cables and plugs were changes 1½ year ago and rotor axle free movement checked and found OK. I better have a look inside to see if things are OK. I can at the same time check the vacuum capsule function...

I now have a properly long to-do list, thanks for your inputs. If you have more, don't hesitate to come forward with it...

Leo

 

A 'backfire' in the intake system will generally sound like a small, muffled explosion or pop. This suggests lack of fuel/weak mixture. If it occurs when the the engine is cold or just partially warmed-up then the Coolant Temp Sensor is a possible culprit as it is the primary player in mixture enrichment until the coolant reaches about 170ºF or so. One would expect a hesitation or flat-spot on acceleration to accompany the problem, typically immediately preceding the backfire in the intake.

Since all cylinders would be equally starved for mixture the engine may well remain smooth-running during all of this. That is, no sensation of misfiring/roughness as you'd feel if (let's say) a couple of spark plug wires were unplugged. The engine may cough (the intake backfire!), bog down, wheeze, and lose power....but you won't feel a cylinder-to-cylinder roughness type of thing.

If all this occurs on a warmed-up engine then the CTS is probably not the issue.

Cheers
DD

 

HA,

If you have intake spit back, and the air filters off. you may get the V12 fire belch. It happens, and will attack the shocker tower paint.

Been there.

I now suspect the fuel load vac/electric switch is inop. It richens the fuel as vac drops as throttle opening takes place, thus allowing a smooth increase in revs. Much like an accelarator pump in a carby.

It is generally located near 5A spark plug, intake runner.

There are 2 versions, and we get this one.



As to why you get issues at light throttle, not sure about this item, but maybe a number of things all add up to one problem.

 

Good evening

After coming home from work I decided to list up the inputs you had supplied

1. TPS output not smooth
2. Throttle syncronisation not OK
3. Faulty vacuum lines or vacuum line connections
4. Ignition advance vacuum capsule failure
5. Ign Amp shielded wire fault
6. Ign Amp condensor fault
7. Ignition timing check
8. Coolant temp sensor @ B bank thermostat
9. Air temp sensor. Plug is not secured by a clip wire anymore.
10. Dizzy internal parts failure
11. Fuel load vacuum switch @5A, light blue. Found it. Will ROM tell me how to check it ?

Re.: 1. TPS: OK in my opinion

Re.: 2. Throttle sync. Checked. RH throttle still had original rubber bushing at throttle axle, while I had changed LH to a brass bushing. RH throttle opening was slightly retarded, so I mounted the brass bushing I had already made.
I took her for a drive of maybe 10 miles, and it went smooth. Around 24°C ambient.
I stopped, shot down engine, polished the inside of my my windscreen, started again without any issues and after maybe 100m I got severe backfiring (closer to bang than poof) and no power until I floored it. This was bad for maybe 500m and completely gone after around 5 km.
I did another and longer stop enjoying a Coke in the evening sun with a buddy. Maybe 40-50 minutes. Engine started well, idled well and drove well for 3-500m, then it hesitated and made a series of small poofs. Maybe 200m later it was gone. That's when I started doing a video, but it just shows normal running...

I'm starting to think vapour pockets in engine bay fuel lines. Being a 1977 my fuel filter is in the engine bay, and I'm not sure how old it is. Maybe I should just mount the spare I have already bought. If flow is restricted in the filter, eventual vapour pockets will take longer to be eliminated. Any thoughts ?

 

Fuel filter, every 5 years, always.

Is the backfire though the Inlet, or a BANG out of the exhaust???

When was the last time it had a Redline run??

 

Backfiring is on inlet side as far as I can hear, also the more violent ones. Most of the time it's just puffing combined with zero power until I lower right foot a bit more.

Last redline run ? Yesterday
And the day before as well...

It changes around 5800 RPM at kickdown, I'm not sure if that if that is normal or too low ?

I will change fuel filter now, thanks. Later I can decide if I should take the effort of moving it to the boot/trunk.