AJ16 intermittent misfire

[cjnsharpe124]

Thanks for the responses. It does make sense that on light throttle air is being sucked back into the cylinders causing the misfire so I am hopeful that fixing them will solve the problem. At the moment I am tending towards getting them repaired. The other options are a) a second hand unit but most of those on ebay already have cracks or b) a stainless steel fabricated unit which looks great but has a disadvantage of cost, heat dissipation and most are cat delete which I'm not convinced about for a road cruiser. Also re-fitting the existing manifolds should be easier than any replacements. I have asked a specialist company for a quote and confirm that the manifold mating faces won't be distorted during the repair. I will update on how I am getting on.

[JimMann]

Whilst browsing another forum (JOC) I noticed that there is a post in there from someone who has had a batch of AJ16 exhaust manifolds created in stainless steel, the pictures look like an excellent piece of fabrication. The cost is listed as £599 to include UK P&P.

For the XJS cars, the following is noted in the description:

"AJ16 exhaust manifolds in 304 stainless steel to suit all X300 Jaguars in left or right hand drive, and with additional small modification, potentially fit cars like the XJS 4.0l"

Contact details are given as: David Austin, email for further information - Jagrow.uk@outlook.com NB Lots of pictures on the JOC site, but not able to copy across.

Jim Mann

[greeban]

I used this guy who runs a company that specialises in welding cast iron amongst other things: Peter Palmer <pp@ciws.co.uk>

https://www.castironwelding.co.uk/about-us/

Peter is a great guy, really knowledgeable and he will happily give you a tour of his factory!

Based in Leicestershire.

[cjnsharpe124]

Thanks for the replies. Update - I contacted one specialist welding company earlier this week who estimated about £500.00 for each manifold repair. This included welding, grinding, crack testing, machining, and bead blasting but it is a lot of money at £1,000 in total. I managed to find a more local company (Suffolk) who said it can be done but didn't include machining and I don't have costs from them. I need to get them fixed but a bit more research is required.

I am not yet 100% convinced that this is the cause of my mis-fire. I think the theory is that air is drawn into the manifold from outside through the cracks - presumably on the piston down stroke while drawing new fuel/air into the cylinder with the exhaust valve still slightly open - and as this now air-rich exhaust gas passes the lambda sensor it registers a weak mixture so the ECU over fuels causing a misfire. This sort of fits in that a) the mis-fire is noticeable at tick over and light throttle and not under acceleration when the throttle demand might override the lambda reading and b) the exhaust gas smells rich at tick-over when warm. The reason I am not 100% sure is that there are many posts about cracked manifolds but very few include an associate mis-fire.

[poprivet]

I had mine bronze welded and re-faced by a local engineering firm in 2003. Still working fine without any cracks. I can't remember the cost but was likely just a few hours labour.

Likely caused by a blocked sump filter resulting in lean running, this was on a pre-facelift so different fuel tank system but very similar manifolds.

If you are searching for manifolds remember the XJ40 (XJ6) was the same so may help to widen the search. There are some on ebay right now so may be worth looking.

[cjnsharpe124]

I have replaced both exhaust manifolds with good used items sourced from ebay. I painted them and re-fitted with new gaskets, donuts, extended brass downpipe fixing screws, new bolts etc and all looks good (heat shield also cleaned up and ready to fit).

But it has not solved the intermittent and random hesitation/misfire problem.

On startup from cold and no throttle the engine runs smoothly but after a few minutes with the coolant at only 20 deg C the random misfire appears (i.e. it cannot trace to one cylinder). This tells me it is not the temperature sender because the ECU is in Rich mode and neither is it the lambda sensors because they are not instructing the ECU at this stage. Therefore it must be fuel or incorrect spark timing. I have already changed the Crankshaft Speed Sensor and then re-fitted the original to check and no change. So perhaps fuel delivery.

The AJ16 Misfire post from a_romeo_75491 for a 1995 Celebration (same year as my car) say they replaced the fuel pump and that fixed his misfire. I have replaced just about everything else (including fuel filter, fuel pressure regulator, injectors) so wondered if the pump is not working properly. I would have thought a faulty pump would manifest itself as significant hesitation under heavy acceleration and/or lean running which I don't have. Research says this is a PITA job so before I go down this route I was wondering if I can test the fuel pressure in any way? Any advice?

[cjnsharpe124]

So I replaced the fuel pump (see separate post) but it has not fixed the engine hesitation / misfire.

I fitted a new intake air temperature sensor on the basis it was the only sensor I hadn't changed.

I retro-fitted the original crankshaft speed sensor bracket and sensor in place of the Andy bracket and new sensor but no change. I checked the plugs again, all ok no obvious oil fouling or over-heating.
.
I removed the ECU (nearside footwell) and this looked very clean and there was no corrosion on the pins.

My 1995 MY car has an OBDII port but as noted previously modern scanners have been unable to communicate with it. So I bought the oldest basic OBDII scanner I could find (2011 D900) and this can read some data. According to the scanner there are no stored DTCs or error codes.

It can read some live data live data including fuel trims, engine temp, engine speed, air intake temp, engine timing and some others.
What is interesting (and assuming the data is correct) are the fuel trims.

At 2500rpm the
STFT B1 is in the range 11% to 19%
STFT B2 is in the range 15% to 25%
Both Long term fuel trims are 48% and have not changed over the past couple of weeks since I started using the scanner.

At idle both STFTs drop to 5% to 12%

I thought STFT were supposed to be in the region -10% to +10% so the readings appear high if correct. I thought these were determined by the feedback from the Lambda sensors but these are new from well respected supplier and club sponsor.

As described previously in the post I have changed just about everything I can think of. Still confused and welcome advice.

[a_romeo_75491]

I posted the earlier comments on a misfire which I cured with a new fuel pump.
More recently I had a re-occurrence of the same problem whilst in France. This time however it was a failing coil pack which I cured by having 6 new coils fitted. Hey presto, runs as smoothly as it should.

I now have a failed drivers door latch. The handle lifts but to no avail. I shall remove the door card tomorrow and see what the issue is.

Drive serenely
David

[cjnsharpe124]

Thanks for the reply a_romeo_75491. Interestingly it was on the basis of your post about a new fuel pump fixing a misfire that I replaced the one in my car but unfortunately it did not work for me. I have also replaced all six coils with new Lucas items so it shouldn't be that.

I am pretty much out of ideas now so I am thinking perhaps I should take it to a Jaguar specialist who has diagnostic equipment to read what is happening in real time.

Can anybody recommend such a specialist in the Suffolk or wider East Anglia area?

Thanks