So, last night I was driving home, zipping along just fine at about 70mph. For once, I was not in Engine Failsafe mode, and was enjoying the full power of my V-8. At some point I noticed that I was losing speed and gave it some gas, only to get a knocking/tapping noise from the engine compartment. I immediately Took my foot off the gas and coasted over to the shoulder; by the time I pulled over, the engine had shut itself off. Until then, no dummy or warning lights came on. The engine cranks, but will not start. It cranks somewhat slowly, and there is clearly resistance, so I don't think its a broken timing chain. I had the car towed home, as no shops were open on a Sunday night, and will have it towed to my regular mechanic tomorrow.

If it had just shut off without warning, I'd be inclined to think it was the fuel pump or some other benign problem, but that knocking/tapping sound has me really worried. However, there is no knocking or tapping when I crank the engine. Any thoughts on what I might be looking at here?

 

Don't think about her till the mechanic can take a look at her and grab codes.

 

Let's hope it's not mechanically serious or there quite possibly won't be codes.

 

Did you take a look under the hood for anything obvious?

 

I pulled the dipstick to look for any signs of coolant intrusion, but saw none. Couldn't spot anything wrong anywhere.

 

Hope you get it figured out on the cheap. But just putting this out, I had a Cat fail once and it sounded just like you are discribing.

 

Well, it took a while, but the mechanic determined we needed to replace the entire throttle body unit. It had been throwing that code for over a year in connection with the Engine Failsafe Mode, and I was putting off replacing it because of the cost, but I guess it finally gave up the ghost. Still, its better than internal engine damage...

 

Ebay has some great pricing on remanufactured TBs, just FYI.

Sorry just checked, doesn't seem to be any up on auction as of right now.

 

The mechanic found a remanufactured one locally, with a 6 month warranty (I think), for $650. I'm sure I could get a better price if I shop around, but I'm getting tired of renting Zip cars every time I need to leave the city

 

Zach do some searching on this forum. Someone found a company that will rebuild your throttle body. Much cheaper and you are sure to get the correct part because it came from your car!!
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Spoke too soon -- apparently there was valve damage to the No. 1 piston, which would explain the tapping noise I heard when I tried to give the car gas before letting it coast to the side of the road. It seems that the throttle body was an issue, as the car now starts with the new one (albeit slowly, with no compression in the no. 1 cylinder), but now the mystery is whether one problem had anything to do with the other...

 

Zach;
With your model of car the timing chain tensioners might have let go and the Jag IS an interference engine. So that kind of damage will happen. No the throttle body has nothing to do with this.
Were you getting any rattling noises on start up before this? Jaguar has gone thru three different tensioner revisions before really fixing the problem.
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No, there was no rattling noise before this. The first time I heard any noise was after I lost power on the freeway and tried to accelerate. I suppose the tensioner(s) could have gone bad suddenly?