Terrible engine knocking sound - please advise

[dave77]

Some background: A few weeks ago my 96 Cobra stopped circulating coolant and overheated badly. It had lost a lot of coolant and oil. I took it to a Ford dealer and they said the compression was bad and the engine needed to be replaced. I got it back and changed the oil (always use Mobil 1 5W30) and flushed & refilled the coolant, and it started running normally again. It did not appear that the oil and coolant had mixed at all (oil in coolant or vice versa). I did a compression test and all cylinders were between 165 and 180, which seemed normal to me (but I'm no expert). Up until last weekend it seemed fine, and then this happened:

The engine started having a knocking sound at part throttle and briefly when letting off the gas. It was worse at some RPM than others, and gradually got louder and happened more. From the frequency I could tell that it was happening in one cylinder (a few times per second at idle). It was quite loud under some circumstances, and I could feel it in the gas pedal. I figured it was a sticking valve. I was 100+ miles from home, so I started driving home avoiding the RPM and throttle ranges where it was bad. I made it most of the way back, then it started doing it continuously and very loudly at all RPM and throttle positions, and didn't idle as well. I had it towed the rest of the way home.

What do you think the sound is? My guess is it's caused by a sticking exhaust valve, and the sound is pre-ignition early in the compression stroke. Can the piston head hit a stuck valve on these engines?

Does anyone have experience rebuilding these engines? Do you know of any places I could get the heads rebuilt? Is it possible to simply un-stick a valve? I have a friend who is willing to help rebuild it and has some experience. Any suggestions on how to proceed? Do I need to pull the engine to remove the heads?

- Dave

[norcal cobra]

wow my name is dave also and i have a 96 that is having a similar problem knocking is no quite as bad as yours gonna get some frpp heads and cam kits soon though let me know what the problem is if you find out soon

[Black Snake]

I’m lost, did the dealer replace the engine and now you hear knocking from the replacement? Or did the dealer say you needed a new engine, you didn’t have it replaced and now the original engine is knocking? If it is the replacement then go back to the dealer, it should be covered under warranty. If it is the original engine then it sounds like you starved the engine of oil when it overheated and you more than likely burned up a bearing.

[Cobra-R]

Generally knocks are lower end, ticks and higher pitched noises are from the top end. It's possible it's a stuck valve, but I am afraid I have to side with Black Snake in it's something a little more serious.
We wish you the best. :thumbsup:

[dave77]

To clarify: I did not have the dealership replace the engine since they wanted several thousand dollars and I wanted to see if anything else could be done.

A couple other people also suggested that it was a bearing (a spun rod bearing), so I'm beginning to think that's what it is, and not a stuck valve as I originally thought. Any advice on how to diagnose and fix that would be appreciated.

- Dave

[tcrews]

A spun bearing is going to need a rebuild. Pull the engine, buy forged components (no since replacing the stock crap with stock crap) and go for it.

The block may need to be machined, probably good idea anyway and bore it to .020 over. Parts will probably be in the $2000 range and labor close to that as well, more if they remove and re-install the engine.

[smashedheadcat]

I just went through with the harshness of a spun rod bearing. The car sounded like two hammers beating the sh!t out of eachother. That's the best way I could describe it. My oil pressure fell off for less than a second before I shut it down. And then a start up at a peformance shop for comformation (few seconds) and then once again before I tore the car apart. So I'd say a total of about 15 seconds of total run time with a bad bearing. My crank shaft was toasted. It's up at True Blue Performance if you wanna take a look at it. Anyway, this a costly rebuild. Most any shop that you take the car to will have to do a full rebuild, just to make sure everything escaped unharmed.

Here's what you're gonna need most likely:

Crankshaft (this is a big maybe, but if you drove it that long, and it ended up being a rod bearing, there isn't as much "metal" as there is on the mains) $600-700

Gaskets- Pricey, Head gaskets alone are about $100
Oil Pump- $50
Bearings- forget the cost, but around $100
Main Studs- the "TTY" bolts have to be replaced -ARP $90
Head Studs- Also, need replaced (TTY)
Rods/Pistons- if you want to (recommended), are around $1500 depending on what you want. I got mine from www.seanhylandmotorsport.com
Machine Work- $400-500 most likely.

This is all a rough estimate, but it's definately pricey. That's no labor, that's no "Remove and Install" labor or anything.

[Big Daddy]

Josh,

Are you back in the sandbox?? Are you going to be bring some of the boys home?? Hope you're ok and we can get together at the drag day in May. :D :D :thumbsup:

[smashedheadcat]

Yes Greg, we poked our head in for a few days. I'm not there right now, but I'll be there again tonight. My computer usage will disappear most likely in a few hours. :( Drag day is looking to be outta the picture (july-ish)

[Ken 01-Cobra]

Originally posted by smashedheadcat
Yes Greg, we poked our head in for a few days. I'm not there right now, but I'll be there again tonight. My computer usage will disappear most likely in a few hours. :( Drag day is looking to be outta the picture (july-ish)

Argh! That sucks! Oh well...maybe you'll be here for the fall drag day! ;) Good luck with the missions!

[smashedheadcat]

Finally got to a computer again. We've been busy bee's. I also heard that our plane made it on the news:D. Yippidy doo!! don't I feel cool :cool:. I'm not sure i'll make the drag day, but if there's any chance, i'll be there. We'll just have to see, but it probably won't happen.

[Big Daddy]

Originally posted by smashedheadcat
Finally got to a computer again. We've been busy bee's. I also heard that our plane made it on the news:D. Yippidy doo!! don't I feel cool :cool:. I'm not sure i'll make the drag day, but if there's any chance, i'll be there. We'll just have to see, but it probably won't happen.

When I saw a C130 made the pickup of the POW's I was wondering if that was your plane or not. So now you're famous! :D ;) Keep your head down, I don't want you getting hurt before you get to enjoy that car! :thumbsup: :thumbsup: