Engine falters/stalls on selecting reverse

[Davidqhog]

Not sure where to put this query but as it started after a gearbox rebuild, I'll try here.

XJ8 4.0 Sovereign 1999, 155,000 miles. The gearbox had a fit of pique a couple of years ago (30,000 miles ago) and had to be rebuilt by a company in Gloucester. After I go it back, I noted that going into reverse caused the engine to falter (drop revs) and sometimes stall. Took it back and was told, after diagnostics, nothing wrong, just needed running.

So, it still does it. Sometimes, it even stalls in 'D' at low speed. I can't see any obvious issues and think the air-cleaner assembly is intact and seated safely on the throttle. It's much worse when the engine is hot - sitting in traffic jams I have to flip it out of drive or it will stall if moving slowly.

Any advice gratefully received!

[Orkney]

Hi David,
Although my Daimler is supercharged I had exactly the same problem when the elbow seals on the supercharger failed. It became a pig in traffic as I had to do like you and knock it out of Drive and then time putting it back in to move off with the traffic. I don't even have the same gearbox as you (Merc derived on supercharged cars) so that shouldn't figure in the equation. Sounds very much like unmetered air entering somewhere after the MAF sensor. To detect my leak I sprayed Easy Start around all kinds of places with joints with no effect, until I squirted some on the Supercharger elbow, when the revs rose each time. I realise that yours is normally aspirated, so that place doesn't exist, but the same process might lead you to the place where the revs rise after a squirt.

Ian

[Davidqhog]

Hi Ian,

Thanks for the reply. I too have a Super V8 (2001, 109k miles purchased earlier this year) which exhibits none of these problems although I did note when I replaced the air filter with a K&N one and cleaned the air ducting that there was some duct tape on part of the pipe presumably to cure a similar problem (note to self: get new air duct!).

I'll try your idea at the week-end and see what happens.

DQ

[J44EAG]

Look for rotten small vac pipes around the throttle body area to the pressure plunum stub pipe.

Leaks around any part of the induction system can seriously upset both idle, part throttle and full throttle response. Incorrect vacuum or pressure values can lead to an unstable engine especially at idle. Placing load from a transmission onto an engine if values are not correct can lead to stalling.

It is also worth noting that a filthy throttle body throat and butterfly plate will affect idle quality. Over the years a quantity of crankcase oil vapour builds up on these areas and when severe can lead to stalled engines. A simple clean out with soft cloth and carburetor cleaner can work wonders. That is where I would be looking as a first point followed by the vac pipe condition.

I`ve covered some of the vac pipe issues in my "The 19 day glitch" article in the August 2016 edition of our magazine under the X350 pages. Plenty of ideas there for consideration.

Mike K

[Davidqhog]

Hi Mike,

Thanks for that. I've cleaned the throttle out a few times - it does get grotty. I'll also check out the pipes around the throttle body too - at 17 years old, they may well be suspect. And thanks for the prod to the X350 article. I may have missed it as it's X350 not 308!

Plenty to check out anyway! I'll report back.

Regards,

DQ