Hello guys;

Car has been running beautifully lately...until this afternoon. Parked it, went to a 90 minute lunch meeting, and;

- on initial start-up attempt, at the point when the engine would normally fire up, I heard one short, percussive release of air. Like air under pressure being released or blowing through something. Sound lasted for 1/2 second. I released the key, abandoned the start-up and wondered what the heck that might have been.

- on following start-up attempts, the engine tries to fire up and there is a whistling sound that rises or decreases with engine speed as I try to keep it running. Very, very rough. A couple of times, I could get the engine to gain some speed and got the revs to 2,000 or so, and it ran much more smoothly...but when the throttle is decreased the whistle returns, the running gets very rough and it wants to die again.

- Slight smell of gas or gas vapor, not too bad.

- No codes are being thrown.

Standing there in my suit, downtown, I popped the hood but didn't see any obvious problems. Based on the symptoms, seems like some kind of vacuum line or connection blew out or off. Would you agree? And why would that happen?

Any ideas on what to check first? I'll get back to the car later this evening for another look, but it's parked on a downtown street so I 'll have a limited toolkit...trying hard to avoid a tow.

Frustrating...it was running so well!

 

Don't know whether you get this kind of thing happening in the US - any chance some 'joker' has blocked your exhaust tailpipes?

Graham

 

Sure sounds like a vacuum leak.

 

AN UPDATE, and a GOOD ONE...

I was able to get back to the car with a small toolkit this evening, and my initial suspicions were correct.

The short "percussive" noise I heard on the first start-up attempt was the sound of my brake booster vacuum line and part-load breather tube connections being blown off the induction elbow. Both were completely off. So that created two huge vaccum leaks, which made for the very rough running and engine stop on the subsequent start-up attempts.

Reconnected both lines, and now she's running perfectly again. And I am quite relieved.

Someone please educate me: what would cause a sudden burst of backpressure in the induction system that would blow off vacuum lines? I would like to NOT repeat that little trick again.

 

That is the most unusual series of events I've heard of in a while. I hope someone has some ideas, I'm stumped.

 

Backfire.

Very rare in today's world of fuel injected cars under normal circumstances. Perhaps you have a leaking fuel injector.

 

Yes, I would guess a backfire also, unless someone fitted a supercharger while your back was turned . But usually an intake backfire is caused by a lean condition, and an exhaust backfire is caused by a rich condition.

If it was an air leak in the intake, I would expect codes.

I have seen an old post where Brutal discusses a 'lazy' MAF causing intake backfire upon acceleration...but your situation was at startup.


With that reasoning, my money would be on a clogged fuel filter or a fuel pump beginning to die and hesitating on startup or a fuel pump relay starting to go funky.

 

I agree, logic would point to the air in the intake area being rapidly heated, expanding and blowing out the connections.

I understand how exhaust backfires happen...but how do intake backfires happen? How does the ignition work its way upstream?

I'm going to treat this as a one-time fluke, until the car tells me otherwise. My fuel pressure has been spot-on, and I've been monitoring my fuel trims for the last month or so, ever since I had the entire intake system off and reinstalled, and all trims have been in the 4 - 8% positive range.

 

It must have been an intake backfire to generate that much positive pressure.
I THINK they happen when the intake valve opens and there's still combustion in the cylinder. In old carburated engines of my vintage that would produce an exciting 'spit' from the air intake because the inlet tract was full of petrol/air mix. As Plums points out that's hard to do with an injected engine unless the inlet valve stuck open.
Is it possible to have a 'lazy' injector going off late in the intake cycle so the mix is very lean and still burning when the exhaust cycle is completed - I really don't know.
What about the dreaded tensioners? Why did you take the intake to bits?
I think your Just Nora (just ignore her) approach would be my choice!

 

I replaced the secondary tensioners just after I bought the car (and just in time, too, as detailed HERE). No problems there.

I had the intake system off to replace the knock sensors, and also had the fuel injectors sent off for ultrasonic cleaning and flow-testing, as detailed HERE. So the fuel injectors should be fine. And truthfully, I had everything off to learn a few things, clean it all up, and understand it more completely.

The car has been running quite beautifully ever since that last bit of work...the best since I purchased it last summer. What a joy this car has been for me (except for the occassional sinking, sick feeling...as experienced yesterday in that parking spot downtown). But I was quite pleased (and proud of my newfound skills) to diagnose and fix the problem curbside, later in the day, in about 15 minutes.

You guys are teaching me something every day!

 

It's of great significance that this occurred during a start attempt. An intake backfire requires two components - vaporized fuel and an ignition source. It's quite common to have significant raw fuel and vapour almost 'stagnated' in the intake tract during start, especially with a cold or almost cold engine. What's usually missing is the ignition source. 'Old school' engines depended on a single coil and a distributor rotor to send the spark to the correct plug at the correct time. A common error when replacing the high tension plug wires was to swap two or more of them, frequently causing a plug to fire when the intake valve was still open which lead to popping, banging and flames shooting out of the carb. Great fun if it was someone else's car.

You mentioned having had some of the engine apart. Is it at all possible that you've crossed some ignition wires and have got the firing order incorrect? Depending on the sequence, aside from a slightly rough idle, the driver may not even notice anything.

 

Thanks for the response, but no...I don't see any way the firing order could be off. The harness to the coils is designed in such a way that one would have to unravel it to get the order mixed up. Just not possible here.

The car has been running beautifully BEFORE and AFTER this little episode. So I'm going to file it away in the "that was interesting" file and enjoy the car until something else pops up.

 

I agree, wait and see if it happens again.

Any computer (thinking the ECM here) can have a 'soft' error. It is rare in a well designed machine, but the possibility exists for a very low occurance software bug, or an internal hardware glitch due to electromagnetic interference, radiation in the atmosphere, electrical noise in the power or some unlikely combination of these or other factors that may never repeat again.

 

I'm no Jaguar expert. Like you this Forum gives me confidence to have a go. There's always someone who's been there and done it.
Sometimes they've even fixed it as well!

 

It was 'a shot in the dark' (pun intended) anyway. I agree- keep motoring on and see if it 'pops up' again.