cannot get rid of fast idle/ P0507 failsafe mode

[700SN]

hi all,
Jaguar XJR 4.0SC 2001
i thought i had maybe solved this, but i am quite at a dead end now, hope you can help or give some pointers..
One evening my sierra needed towing about 10 yards on my drive as her battery was flat and needed to move to a safer position.
The drive is flat. i hooked up a tow-rope on both towing eyes, and very tentatively eased the throttle forward in D. i hesitated once just to reposition the sierra's wheels and the XJ was in neutral. (i didn't put her in P as i thought i might have strained the 'box) all was well, and i towed another 5 yards and put XJ in N whilst removing the tow rope. Then i proceeded to drive off and hesitated at the end of the drive, and i saw "Failsafe Engine Mode" come up. it was strange, as i hadn't forced or jolted whilst towing.
The next few days i have done the following:
1. bought a new scanner (Autophix OM 126P) which incidentally, was rubbish as it only worked once then just says "FAIL" when it wants to read the port (big waste of £59.99)
2. the codes P1111 (Intake air temp sensor circuit intermittent high voltage) and P0507 (idle control system RPM higher than expected).
3. Bought and fitted a new MAF
4. Checked for any loose pipes like PCV
5. cleaned the butterfly as far as i could, with intake cleaner -it wasn't really dirty anyway
6. checked the TPS was secure
7. checked the throttle cable wasn't unduly slack or tight
8. checked all fuses under bonnet, boot, and N/S rear fusebox, all ok
9. did hard resets many times
10. took her on a 2 mile run, no clearance of Engine Failsafe mode.
11. did all of this with a 100% charged battery, 12.84V and all terminals clean and tight.

After each reset, Engine failsafe mode comes on immediately (even with ignition on but no running of the engine) and sticks the revs at 1200 after blipping initially at 2500. it is about double what it used to be.

The TPS is fairly new, only 7,000 miles ago.

it's a tad annoying , but as i have lost the will to live with the scanner, as it will not now read, i am reluctant to buy another....

assistance most appreciated
cheers!

[andypryce]

The new MAF will mess up your idle speed for a while until the ECU adapts to the changed resistance in the MAF wire, in that respect I would drive it for a while and see if it corrects itself? Plus send the scanner back if possible as sounds like it’s duff….

[700SN]

Andy, thank you!
May I ask why the new MAF would be a change of resistance in that wire? How many miles would I have to drive her for… it’s a bit of a pain as she tends to “drive herself “ in low speeds and needs a bit more braking .. also the gear changes are not quite right.

[andypryce]

The old MAF wire will have deposits on it, over time the ECU adapts and allows for that, a new wire will be clean and that essentially ‘confuses’ the ECU as the resistance in the wire has changed, but gradually it adapts to the changed resistance in the wire. Exactly same happens if you clean the MAF, eg with contact cleaner. Not sure how long it takes to settle, but would guess within 50 miles or so, others wiser than me may know. If all was well before then,hopefully, things will be fine again once ECU has adapted….

[700SN]

Thanks a lot! Will see how it goes over the next week!

[andypryce]

PS: very jealous of your XR4, had a XR4i in about 1985-1987, loved that car…

[700SN]

hah blimey! that must have been expensive then.... that was when i was driving my '75 JPS 2.0S Capri !

just a quick one... so even after a "hard reset" the ECU still needs to "remember" stuff over 50 miles and build up a new memory...

[andypryce]

Yes I think so, I would think after a week or so it should settle. If you have the old MAF you could always try that again.

[700SN]

Ok.. I slept on it. I then went out again and took the air inlet off and opened up the throttle butterfly carefully with a clean long heavy duty flat head screwdriver, which was a lot better than I’d done before. I thought I’d maybe not cleaned the butterfly enough. So as I could still see residue I totally sprayed carb cleaner around it and worked it with an old toothbrush. Then sprayed again.

I then undid the TPS top screw (if anyone knows what these exactly are, kindly let me know as I couldn’t find a nice fitting torx) I couldn’t seem to undo or access properly the bottom screw but I managed to lever it anti-clockwise by a couple of degrees.

Then I sprayed through the crankcase breather pipe that’s attached to the air intake whilst the whole thing was off. I then sprayed electrical cleaner in the MAF plug and cleaned with a toothbrush.

So after reassembly I started her up….. “engine failsafe mode” immediately. I then turned off…did a hard reset of the ECU by putting the Negative lead on to the positive terminal, for 30 sec. Then leads back on.
Waited for 2mins. Then turned key to ignition on but not start her. Leave 2 mins. Started her up.
Revs 2500 then dropped from the previous 1250 to 900. I let her get warm, and low and behold the revs dropped to 750 but still EML but when she reached optimal temp, all of a sudden “engine failsafe mode” disappeared!
I then took her on a 3 mile run and all was fine. In fact she seemed more positive in response.

Now I’m not sure what I did, but my instinct tends to point to a sticky butterfly. Maybe on towing, the revs didn’t quite match the ECU memory and maybe forced the butterfly in its gunge, or tricked the TPS. Or maybe it was a coincidence that night, the MAF decided to give up - I’ll never know, but the sticky butterfly is my guess.

I would just like to thank you for your responses and wish you a great weekend!

[andypryce]

Well done! Itching to clean my MAF as I’m sure it’s never been done, but a voice in my head keeps saying ‘If it’s not broken….’ !

[700SN]

I’m convinced that that that was a sticky butterfly.
Reason:
The other day I’d disconnected the battery and charged my XJR overnight. Wife wanted to be taken somewhere in the morning so - Hooked her back up, usual high idle 2500 on cold start up when resetting itself.. went down to 1000, (usually I would wait until warm) but wife wanted to go, so I drove off.
Then the tell- tale behaviour started- faster idle. Like the XJR was driving itself. “Failsafe engine mode” comes on…
Cut a long story short..did hard reset - I took off the air ducting around 10 times to try and adjust the TPS, as I thought may have messed it up when I first drove off cold whilst it tried to reset.
One thing I’ve learned is when you look at the TPS head on, there are 2 screws holding it. You need a proper TS20 (not a T20 or Phillips otherwise you’ll ruin the head) and to get to the bottom screw you’ll need a 90 degree attachment. Then you loosen slightly and turn it. Now previously I had info that to get idle lower you turn it anti clockwise. I did this and wow I thought the engine was going to blow up.. so that info was wrong. In tiny increments you should turn it clockwise until the correct idle of around 650 is found- each time switching the engine off in between adjustments.
Normally the screw should sit in the middle, but mine is just very slightly to the left.
She now runs nicely and all I can think of is that hooking the battery up and starting from cold and driving off when she hadn’t quite settled, messed up the position of the TPS or the KAM of the ecu.

I hope that helps someone!