I've been working my way through a myriad of odd driveability symptoms, and what I'm wondering if anyone might know what this symptom means straightaway... When I take off, let's say I go to 100% throttle immediately, the engine will nose over hard and stumble its way forward and as rpms pick up, it clears out runs sort of half-assed. Normally it has great throttle response and gobs of torque rolling on to where it will spin tires at 20% throttle in the wet. Also if I rev it hard, it stumbles and hiccups badly.

So what the hell is it doing? If the car had like a variable manifold valve I would suspect that, but I dont see one anywhere unless I missed it...but Ive done plenum gaskets already too. And a crank sensor. Maybe its a cam sensor problem?

 

Codes? Fuel Trims? Without them, it is like calling the doctor to tell him you don't feel too good!

 

I agree, it's hard to say, but if I had to guess, it's a fuel problem. Either a lack of fuel pressure, or a spectacularly bad batch of gas.

Has this persisted for longer than one tank of gas? Or is the gas really old?

 

what model? XJ8 or XJR. XJR has 2 fuel pumps.

 

Hey guys, its an XJ8 with one fuel pump. It has done this or weirder stuff since I got the car. Which is why I initially suspected a vacuum leak and did plenum gaskets because I knew there was a known issue with them. I thought I saw some smoke around the runners using our Diagnostic Smoke Tester but its hose was rotted through and leaking so it was hard to pinpoint anything.


Fuel pressure at idle is 30-32psi WITH vacuum applied, on a rev it jumps up from 35-40 depending on how I rev it. This has been a persistent problem since I got the car. Ive replaced crank sensor, intake plenum gaskets, cleaned TB off of the car without touching the blade, then cleaning it on the car with a throttle-stick...also new Iridium spark plugs, fresh oil change with full synthetic, added trans fluid (Valvoline Max Life....and the stuff in the transmission now looks horrific. Dark and full of particulate.

t long term trims are flatlined at very close to 0% and short term trims hover + or - 4%...dropping to -8% on a rev and then plus slightly less as the engine decels back to idle. O2 millivolts look good also, both sides are very even on pulsewidth and their mlllivolt ranges are very close.

***NO CODES***

 

Howe does it drive if you are easy on the throttle?

 

First replace the fuel filter- cheap and easy.

 

Agreed. These are often overlooked.

 

I replaced the fuel filter along with the air filter, sorry forgot to mention that...

Normally it drives fine for the most part if youre just putting along...although sometimes I'll go like 20% throttle to pass someone and where normally it would be pulling very good, it's just blah. It comes and goes. Also on a hard acceleration where it does pull pretty good, you can feel it has a rolling surge or stumble..like a coil is cutting out maybe?

 

knock sensors.....?

mine did the same, and got worse as we drove it more....

replaced the knock sensors and it has been PUURRRFECT since

 

Would there be any indications? Could a knock sensor cause the computer to pull timing without tripping a Limited Performance message? That is sort of what it feels like, but if I rev the engine, it does stumble...sometimes it feels worse with higher load.

 

knock sensors are a lean event fuel dump switch, nothing more. Have you heard any audible 'pinking' or detonation from the engine? That's crappy gas....

Try the highest octane rated fuel you can get - this can rule out gas if nothing improves.
The plug gap is 1mm.
With no codes it's tricky to pinpoint, lets recap on your posts
New fuel & air filters
New intake gaskets
New plugs
Cleaned throttle
Oil service

Fuel trims within spec' O2 sensors feedback OK.

Have you cleaned the MAF? or is it a one piece version? I'm familiar with the AJ27 that's held in and can be cleaned. As you don't mention the year model I guess...

Have you an OBD reader than can be left plugged in to capture a code that doesn't set, sometimes an intermittent engine fault can pop up while driving and go when the engine is shut off.

More info = more suggestions

 

Sean, thanks for troubleshooting with me, most don't have the patience. Here you go...

Gas: I always run 93 octane, the highest grade available, and have used several fill stations, and Lucas injector cleaner. Problems occurred randomly both with and without the cleaner present, and with gasoline from several fill stations.

Plugs: It had a set of newer plugs in it that looked pretty good but since I work at a parts house and I can get plugs pretty cheap, I threw in a set of Iridiums that we looked up by application and were gapped in the box, though I verified gap before installation.

MAF: It has an exposed filament, I did not clean it but I verified that it looks clean, and I also graphed its response and curve with throttle position and it seems to match up. Also response is quick, so I discounted it unless I missed something...

That's a good idea though, to datalog it...I forgot that my laptop-based scantool can datalog, or at least Im pretty sure it can. Time to hook it up and go for some driving!

Apologies on year make model, thought it would show up in my signature. 2001 XJ8 Vanden Plas, 4.0 non-supercharged.

 

Datalogging hoping to find a pattern or even a pending code sounds good. Nothing to me says you've done the wrong things or overlooked anything. I also can't see it being a knock sensor. Possibly a bad coil, and high load would be its moment of choice. Are the plug wells clean (of oil/water/...)?

 

The plugs come out 95% clean...a good light tan color, the leaner end of stoich... No signs of hot spots, erosion, discoloring or fuel staining, none of them stink of fuel....Two of them have a little oil spotting that's very light and isolated to the ceramic, but that very possibly could have been done by me, when I did the manifold gaskets and cleaned the manifold thoroughly and intake ports out a bit lightly by hand, no chemicals there...

I keep coming back to a coil dropping out too, but the damn plugs don't read it. They're as even as can be. Maybe something with the cam sensors acting up, causing the computer to doodle around the timing, or valve timing even? This is a VVT motor correct? Thanks for the help

I will datalog tomorrow or Thursday, will post any revelations then

ALSO, I replaced the crank sensor shortly before making this Topic...Very slightly smoother idle, almost imperceptible, no other changes though.

 

You seem to have done all the normal things to fix the problem.
These are just a stab in the dark.
Have you checked the inlet trunk for leaks, the corrugated part can crack and not look obvious, also the resonator, ( 4" long tube sticking out of side) can crack. The cracks might open up when you rev it.
Also check the earth cable, underneath from motor to chassis, connections and cable can corrode, might drop voltage when motor moves.
Could check the ecm for bad connection, or water leaking on it.

 

I suggest testing the fuel pressure under load- Install a longer hose on your fuel pressure gauge and tape it to the outside of the windscreen. The pressure can be fine at idle since the regulator will compensate, then drop off under flow.
On my NA XJ8, the fuel pressure inlet screen was partially plugged. I think it was dropping some of the crud when turned off, then re fouling the screen when re-started, further complicating the diagnostic process. If the fuel flow is dropping off under load, the fuel trims will go haywire while it is happening - assuming you are not WOT and in open loop control. So, try the suggestion of charting fuel trims while driving.

 

The datalog didnt work right today, I had it set up for Fast Polling, and the Jag ECU isn`t compatible with that so instead of recording 5-6 PIDS per second, which is as fast as I can get it, it only managed 1.3, so I had to switch it back off and my data was useless because I didn't catch it til after. So gotta try again tomorrow. I do notice a trend of very high fuel trims under load and on decel after being on the throttle. The short-term fuel trims were as much as 25% to either the negative OR the positive, depending on what I was doing. IIRC it was calling for positive fuel under load, and negative fueling on Decel.

 

I definitely need to test drive it with a fuel pressure gauge because most of the time it runs fine. But sporadically it has a bad loss of power. Also rapid transitions from no throttle to 100% throttle cause it to drop rpms, shudder then climb up shakily. Taking off and going straight to Wide Open Throttle, it barely lunges forward and makes this noise that sounds like an engine that can't breathe. Whereas a smooth transition from zero throttle to 100% throttle, the car picks up and starts to accelerate fairly smooth, with a few rolling surges and the engine itself feels shakey...like, running on seven cylinders shakey. It has a weak or dead motor mount though so I don't know with certainty that it does have any sort of miss.

 

Hmm throttle body maybe?