Knocking noise only during cold start?

[erykv1]

Hello ,

My 2001 XJ (4.0L I6), started making a really loud racket during cold starts every morning before work recently. When the jeep first fires up, it sounds like its knocking. However as soon as I give it some gas it goes away and is gone for the rest of the day. I have eliminated the possibility of rod knock as it immediately goes away. Oil pressure on the jeep is also fine.

I am thinking that it is possibly my flex plate on the way out. However, all the flex plate trouble shooting threads I've read have talked about a constant noise that doesnt go away. Is there something else I could be overlooking?

Thanks in advance for any advice / input

 

[ehall]

Probably valve rattle. Do you have a helper? Get a mechanics stethoscope and listen to the engine while your helper starts the engine (do not rev). If its the valves you just need to improve top-end oil flow. You can try running some Marvel Mystery Oil for 500 miles or so before oil change, see if it helps anything. Otherwise I would probably remove the valve cover, clean the pushrods with some Berryman's Carb Cleaner and spray a little into the lifters, retorque the rockers and run it.

 

[erykv1]

Is valve rattle supposed to sound like an engine knock? It almost sounds like its coming from the lower bellhousing / trans tunnel area when I'm sitting in the drivers seat.

Thanks for your input.

 

[ehall]

The reason I say it is probably top-end rattle is because (1) its very common and (2) increasing RPM increases oil flow which is probably overcoming whatever resistance problem you have and getting enough oil on top to lubricate the moving parts

It could be something else yeah but it would probably keep coming back at low RPM.

Get a mechanic's stethoscope and listen. Three foot section of 1/2" hose in your ear is also pretty good at isolating sharp noises

 

[Digger87xj]

check your oil level and condition lately?

 

[JeepityJeep]

Catalytic converter

 

[erykv1]

The reason I say it is probably top-end rattle is because (1) its very common and (2) increasing RPM increases oil flow which is probably overcoming whatever resistance problem you have and getting enough oil on top to lubricate the moving parts

It could be something else yeah but it would probably keep coming back at low RPM. Its just weird that the problems are completely gone until the next morning.

Get a mechanic's stethoscope and listen. Three foot section of 1/2" hose in your ear is also pretty good at isolating sharp noises

Ok, I hope its top end rattle and nothing serious.

check your oil level and condition lately?

Oil levels look fine, almost due for a change though.

Catalytic converter

Interesting... would it make a knocking noise?



anyhow, I'm going to pull the pan off tonight and see if its anything clogging the oil pickup. Will report back.

 

[JeepityJeep]

My catalytic converter did exactly what you reported during start up. As soon as you start it up first thing in the morning, just crawl directly under the converter to check The way my converter failed also significantly impacted my engine performance under load.

 

[erykv1]

My catalytic converter did exactly what you reported during start up. As soon as you start it up first thing in the morning, just crawl directly under the converter to check The way my converter failed also significantly impacted my engine performance under load.

It sounds really really bad, yours sounded like engine knock? I wont rule this out.

Thanks.

 

[JeepityJeep]

Mine sounded nasty, for the first five minutes or so, but I would describe it more as metal on metal knocking. No it didn't sound like pre-detonation engine knocking/run-on.

Either way, tomorrow morning just crawl under your jeep just after you initial start it up in the morning... follow the noise

 

[ehall]

My Cadillac had some cold-start converter rattle. When it was cold, the honeycomb brick would shift around inside the can, then as things heated up and expanded it would be immobilized. Didn't respond to throttle though, it took time to heat up before it would stop moving around. It also made clunk noise when shifting into gear, because of the movement.

 

[trippled]

Does it run rough? Mine does this and usually misfires as well, but its intermittent. Pretty much narrowed it down to a lifter bleeding down. Its done it for years now. If cleaning doesn't fix anything as stated above I'd just run it.

 

[erykv1]

Mine sounded nasty, for the first five minutes or so, but I would describe it more as metal on metal knocking. No it didn't sound like pre-detonation engine knocking/run-on.

Either way, tomorrow morning just crawl under your jeep just after you initial start it up in the morning... follow the noise

Thank you for your input. I'll check it for sure

My Cadillac had some cold-start converter rattle. When it was cold, the honeycomb brick would shift around inside the can, then as things heated up and expanded it would be immobilized. Didn't respond to throttle though, it took time to heat up before it would stop moving around. It also made clunk noise when shifting into gear, because of the movement.

Thanks, I'll be sure to check in the morning.

Does it run rough? Mine does this and usually misfires as well, but its intermittent. Pretty much narrowed it down to a lifter bleeding down. Its done it for years now. If cleaning doesn't fix anything as stated above I'd just run it.

Yes it runs rough, but I attribute that to a faulty fuel pump check valve not keeping enough pressure in the fuel rail. Its rough enough to feel it in the steering wheel. But as soon as I give it gas all is well. Super weird.

I will check the flex plate bolts tonight and go from there.

 

[Madmanmarty]

How about 4.0 lifters?
They are notorious for making a racket at start-up.
My wife's grand was making some noise on cold starts.
I added Engine Restorer at the next oil change and the noise disappeared.
Give it a try. It's only $10.

 

[erykv1]

thanks guys for the input so far.

heres an update:

Last night I got under it, gave the cat a few taps... sounded like the knocking noise. Tightened it up for good measure. I also checked the flex plate and it looked good (didnt think this was it, since the noise went away when it was warm).

Sooo... this morning I turned the jeep over, and instead of giving it gas to get the knock to go away, it kept an eye on the oil pressure gauge. So that rules out the cats.

As soon as the oil pressure rose to operating pressures at idle, the noise went away. I'll check the lifters next, but perhaps the oil pickup is obstructed?

 

[Digger87xj]

what filter do you have on there?

I suggest using a wix or napa gold. Fram and other cheapies will allow oil drainback.

 

[erykv1]

what filter do you have on there?

I suggest using a wix or napa gold. Fram and other cheapies will allow oil drainback.

I'm running a mobil1 oil filter. Same horrible noise this morning, but it still got me to work. :looney:

 

[mtbxj87]

what brand oil filter do you use?

 

[nicpaige]

How many miles? Did this start all of a sudden or gradually get worse over time? Tightening your converter wouldn't help the cause of your issue if it is in the converter. You would need to replace or eliminate the converter to get rid of the chunks of material beating around in there. I have had this very thing happen to me on a Mustang and didn't figure it out until after I replaced the bearings. What you describe sounds like good old 4.0 piston slap to me.

 

[mtbxj87]

sorry for the double post, previous post didn't show..