Greetings, After replacing plugs and fuel pump/filters I now have a loud thump at the top of the engine. Any thoughts? Thanks

 

A "miss" comes first to mind. Check the coils and airbox clamps. List the car and year in Avatar.

 

Ummm, what is a plastic thump? When dose this loud thump happen? On start up, under WOT, idle, all the time? Does it match the RPM or speed? Is it constant or intermittent? Sorry for all the questions but if you want help, please be as informative as you can. There are a lot of VERY smart members on this sight that will help you out.

(p.s. I hear the Mods foot steps... you better run to the intro thread and read rules and the FAQ... yikes...)

 

Well thanks for the input, as I'm new to the site here's more details... the engine is a 4.0 no superchager w/ about 130K miles. All of a sudden it was running out of time as if it was firing on half the cylinders. We replaced a faulty fuel pump and its filters, replaced all spark plugs checked all coils all to no aval. We have now removed the cam cover, everything looks pristine and new. The chain is tight tensioner is taunt cranked engine and no longer hear rythmic thumping yet engine still idles rough??????

 

Wow, I was just over looking at these post ( my father's been trying to help me) and perhaps I can help here. its a 2000 Jaguar S type 4.0 non supercharger. It has 135k miles on it. I have been religious about the maintance and a side from the occasional coil needing to be swapped out no major issues. Last December (2010) Car all of a sudden started running like an old stock car ran really rich and rocked like a nascar. I limped it home (8miles) and its been sitting since. I have tried the things with my fathers assistance of course (mechanic ) however the car still is no where close to running smoothly. My gut tells me that its a sensor but I am obviously way out of my league here and looking for some clues. I really appreciate all of your feedback and suggestions!

 

If you're 100% sure it's not jumped a tooth and that it's not any other mechanical thing then use OBD. Check for codes. If some, post them here. If none, watch live data and see if it rapidly (30 secs or so) goes CL (closed loop) as it warms up from cold. Then look at LTFTs (long term fuel trims) at idle and (in P) at about 2500rpm. The 2 banks should be very similar and close to zero (post them).

I suggest an elm327 (USB, probably) - very cheap but also very good. You need a PC or better a laptop as well, which is why they're so cheap (doesn't have its own display or buttons).

You can also look at lots of sensors (ECT, IAT etc) and eyeball the values looking for any that are plainly wrong - the car largely just has to trust them so any wrong ones can cause grief.

 

Vaccume leak.

 

I would hope your father being a mech would have checked all the vacuum lines already. But that can cause hell on these cars. If not, then please follow jagv8's advise. He sometimes speeks over even my head a bit, but knows what he's talking about. lol

After 8 months, I really hope you get your cat purring again.

 

Large and small vaccume leak(s), i.e. intake manifold, air box, etc., not just hoses.

 

Following what I posted about OBD will show whether there's a leak

Going CL and good fuel trims = no leaks.

 

Check that the tensioners have been updated while you are in there.

 

Greetings folks, thanks for all of the input... upon further dissassembly the sorce of the hollow-plastic thumps were found under the right side cam cover! (my hearing is shot) ...the tensiner and secondary chain broke. I'm pulling the chain cover over the next few afternoons to collect the plastic fragments (I have recovered all but one roller of the chain). Any thoughts as to the state of pistons and valves I've no clue as to their clearances?????????

 

Try not to panic but they're probably a real mess. The 4.x is an interference engine.

 

Yikes, that's not good. I'm glad you found the issue, but ouch. IF you don't want to rip into it yet you could use a scope and get in there to check it out. If you don't have one places like Advanced, or AutoZone will rent them to you for a 100% refundable deposit.

 

I wanted to post an update on my kitty's status:
So after tearing it down to the block we were able to see that the valves, seats and cylinders all were in good shape So I have gone to the parts store an picked up the following:
-New head bolts as they are stretching bolts and cannot be reused
-timing chain gasket inner and outer
-secondary timing chain
-tensioner
-new head gasket
-new valve gasket
-waterpump (mine was fine but given the hassle to get to it I figured might aswell throw a new one on to save myself the labor + headache
-all new plugs-iridium
Everything minus the tensioner came in today (tensioner will be here tomorrow am) so we will begin assembly tomorrow night why? Because friday night date with a pretty car is the way we roll!
Okay, so here is my question for you guys ( and yes, ya'll are the most helpful and very knowledgable and I trust you alls opinions/suggestions over the dealer)
1. What could have possibly caused my timing chain/tensioner to fail. My understanding is that these chains are all "lifetime" and that something else caused the initial missfire that ultimately lead to this current mishap.
My thoughts go to the elbow vaccum hose behind the intake manifold, the ignition control soleniod, and even the catalytic converters. I would greatly appreciate your thoughts so I can get this pretty kitty purring again. I'll be posting pictures from this whole little adventure when I am able to post attachments. Thanks again to all of you!!!!

 

The tensioners are a well documented weak point on the 4.0's
It's time may well have just been up.

Let us know how the rebuild goes.