Flexplate or rod bearing?

Hi guys,
I'm new to the forum, and to the "ways of the Jeep". Bought a '91 Cherokee about 3 months ago, 167,000 mi, automatic transmission. It has developed a knock at startup and it is intermittent when running. If you blip the throttle it will knock every time. It does not knock any worse if the engine is under load, and the oil is clean on the dipstick. However, I have not pulled the pan to take a good look at what's in there. I'm really hoping it's either a loose or cracked flexplate since that would be the least costly fix.

My question for the guru's is how do I rule out one or the other short of completely tearing into either the engine, or dropping the transmission? Thanks in advance.

John

Well, you could pull the inspection plate and with a strong light examine/tighten the flexplate bolts to see if that makes a difference.

Easiest way to check, is to check.
Crawl underneath your rig, unbolt the inspection plate (front of bellhousing-two 13mm bolts and two 18mm through bolts with nuts).
The bolts (one or two anyway) will be visible approximately half way between the center and the outside edge. These bolt heads are 15mm (can you tell I just did this?).
Put a wrench on one and try to tighten-if they are loose, they will tighten some before the engine rotates.
To cinch them tight, wedge a large screwdriver in the ring gear teeth.
If you don't locktite them, you will probably have to do it again some time down the road.
All that being said, mine was making an irregular knock while idling in neutral. Once in gear, the noise abated, as the load takes the slack out.
I honestly don't know what a cracked flex plate sounds like.
How's your oil pressure?
Any drastic changes recently? Engine heat up more quickly? Power any less?
Idle the same, or different?

Nick

Nickfast said:

Easiest way to check, is to check.
Crawl underneath your rig, unbolt the inspection plate (front of bellhousing-two 13mm bolts and two 18mm through bolts with nuts).
The bolts (one or two anyway) will be visible approximately half way between the center and the outside edge. These bolt heads are 15mm (can you tell I just did this?).
Put a wrench on one and try to tighten-if they are loose, they will tighten some before the engine rotates.
To cinch them tight, wedge a large screwdriver in the ring gear teeth.
If you don't locktite them, you will probably have to do it again some time down the road.
All that being said, mine was making an irregular knock while idling in neutral. Once in gear, the noise abated, as the load takes the slack out.
I honestly don't know what a cracked flex plate sounds like.
How's your oil pressure?
Any drastic changes recently? Engine heat up more quickly? Power any less?
Idle the same, or different?

Nick

Click to expand...

My son has been driving it lately (I actually bought it for him) but there have been no drastic changes in behavior. Good power, no change in idle or operating temperature. . . not sure about the oil pressure though.

I guess I'll remove the inspection plate and tighten what I can, The flexplate seems like the logical cause. I'm just not convinced that these symptoms are an indication of some serious engine issue, I sure hope not.

. . . maybe I'm in denial.

The oil pressure drop due to rod bearings isn't as drastic as it is for mains.

To rule out the flexplate:

1. Remove inspection cover
2. Get a bright flashlight and a longish flat screwdriver
3. Shine the light up towards the hub of the flexplate
4. Insert the screwdriver between the engine and flexplate
5. GENTLY prise the flexplate away from the engine
6. Note whether or not you see the flexplate separate at/near the hub
7. Repeat at 120* or 90* intervals to ensure a thorough check
8. Reinstall inspection cover
9. Source replacement flexplate if necessary

I have been having good luck with flexplates from Pioneer/Barnes. It is suggested that the flexplate screws be replaced when the flexplate is changed - it may not be necessary, but it's not a bad idea. They are 1/2"-20x.600" "Place" screws - the head design is slightly different from the standard hex head, and for good reason (different loading, needs different clamping ability.) They can be had from the dealership, or you can get a kit from ARP that is actually for a Pontiac V8, but contains six pcs. 1/2"-20x.590" screws that work neatly for our Jeeps (and don't need to be replaced each time, since they're of a better alloy and design. I've been using ARP where possible for years - they cost more, but they're well worth it!)

If you do not see any bluish stuff on the threads (that's threadlocker,) be sure to apply LocTite #242 (or equivalent) when you put the screws in. Torque to spec - it's on my site.

Other things to check while you've got the inspection plate off:

Make sure the torque converter-to-flexplate screws are tight. Torque spec for that is on my site as well, and that can present as a flexplate knocking. If it's loose, remove it, clean the threads with a wire wheel, and apply LocTite #242 (or equivalent.) Replacement screws are M10-1.5x20m/m "Place" screws - again, they are not hex head capscrews! These can be retorqued with the transmission in place.

While you're looking for a crack, check to make sure the flexplate proper stays nice and tight against the crankshaft flange. Loose flexplate-to-crankshaft screws can also present as a cracked flexplate. You'll have to remove the transmission to tighten these.

Use a torque wrench to tighten these screws! Preload is vital. (Preload is what you get when you tighten a screw to a specific level. In many other countries, "torque wrenches" are called "tension wrenches," and this is, in fact, a more proper name for them.)

The engine may be turned using a large ratchet or breaker bar with a 3/4" socket.

easy way to check for cracked flexplate...
if the knock goes away, or changes markedly when the transmission is in gear, its a cracked plate. period.

this is my 92, before I replaced the busted plate. Got worse than this over the following months.

http://www.youtube.com/watch?v=VEcXQ4jjxZI

i had some knocking/rattling at startup (about 2 seconds worth) and when i "tapped" the accelerator just enough to get the engine up to 2500rpms and then letting go (about 1 second just as it peaked at 2500)

Turned out to be the rod bearings... they were FAR gone... even had to replace the crankshaft....

xj88kjetil said:

i had some knocking/rattling at startup (about 2 seconds worth) and when i "tapped" the accelerator just enough to get the engine up to 2500rpms and then letting go (about 1 second just as it peaked at 2500)

Turned out to be the rod bearings... they were FAR gone... even had to replace the crankshaft....

Click to expand...

I'm afraid that's where I am too. I had a mechanic look at it, he pulled a couple plug wires and the sound went away, leading him to the same conclusion: rod bearings. Not really sure how to proceed from here. . . (his suggestion was a new long block @ $3000 installed) It's a pretty clean '91, but will never be worth the cost of going that route. First thing is to pull the pan and assess the damage I guess, then buy a good service manual and have you guys teach me how to become a Jeep mechanic.

I have 25 years experience with rebuilding/repairing farm machinery, but I always left the engine rebuilding to someone who was trained in that area. . . much more efficient use of time. However, the mindset of people in my profession is, "Ah hell. . . it can't be THAT hard."

jmayer said:

I'm afraid that's where I am too. I had a mechanic look at it, he pulled a couple plug wires and the sound went away, leading him to the same conclusion: rod bearings. Not really sure how to proceed from here. . . (his suggestion was a new long block @ $3000 installed) It's a pretty clean '91, but will never be worth the cost of going that route. First thing is to pull the pan and assess the damage I guess, then buy a good service manual and have you guys teach me how to become a Jeep mechanic.

I have 25 years experience with rebuilding/repairing farm machinery, but I always left the engine rebuilding to someone who was trained in that area. . . much more efficient use of time. However, the mindset of people in my profession is, "Ah hell. . . it can't be THAT hard."

Click to expand...

I'm not sure that I understand how pulling plug wires can lead to diagnosing bad rod bearings. Everything in the motor is still turning about like it would be on all cylinders firing. It's not like one piston/rod quits working...it's all attached by the crankshaft....:spin1:

Pulling plug wires eliminates the "hammering" on the rod bearing and will make it noticeably quieter. I have done this many times before. It doesn't always mean rod bearing, but is pretty common cause.

I am actually having what sounds to be the same noise. I had mine down to piston slap or lifter noise from probing with an extension and listening, but for some reason forgot about pulling plug wires. I will be doing that tonight. Thanks for the reminder guys! That being said, piston slap usually goes away or gets better when it warms up. Mine does not. I sure hope mine isn't a rod, I am sourcing and buying parts for a lift as we speak.
-Aaron

jmayer said:

I'm afraid that's where I am too. I had a mechanic look at it, he pulled a couple plug wires and the sound went away, leading him to the same conclusion: rod bearings. Not really sure how to proceed from here. . . (his suggestion was a new long block @ $3000 installed) It's a pretty clean '91, but will never be worth the cost of going that route. First thing is to pull the pan and assess the damage I guess, then buy a good service manual and have you guys teach me how to become a Jeep mechanic.

I have 25 years experience with rebuilding/repairing farm machinery, but I always left the engine rebuilding to someone who was trained in that area. . . much more efficient use of time. However, the mindset of people in my profession is, "Ah hell. . . it can't be THAT hard."

Click to expand...

You could always try the shadetree method of replacing the current rod bearings with standard bearings with the engine in the vehicle. Depending on the condition of the rod journals you might have some success. All you would lose is the cost of the plastigauge and bearings, some sweat and scraped knuckles.

joe_peters said:

You could always try the shadetree method of replacing the current rod bearings with standard bearings with the engine in the vehicle. Depending on the condition of the rod journals you might have some success. All you would lose is the cost of the plastigauge and bearings, some sweat and scraped knuckles.

Click to expand...

joe,
Can you link me to a good step-by-step explanation of how to proceed? I have a Haynes manual and just enough experience to be dangerous.

jmayer said:

joe,
Can you link me to a good step-by-step explanation of how to proceed? I have a Haynes manual and just enough experience to be dangerous.

Click to expand...

You would just follow the Haynes.

Pull your spark plugs (it will make rotating the engine easier), and drop the pan.

Pick the rod journals that are the lowest, remove the caps. You need some hose or tubing to cover the threads, then gently push/tap the rod and piston up and away from the journal. Replace the bearings, use assembly lube, reassemble and torque to specs, then repeat five more times.

If you have a heavily damaged journal your new bearings won't last very long.

joe_peters said:

You would just follow the Haynes.

Pull your spark plugs (it will make rotating the engine easier), and drop the pan.

Pick the rod journals that are the lowest, remove the caps. You need some hose or tubing to cover the threads, then gently push/tap the rod and piston up and away from the journal. Replace the bearings, use assembly lube, reassemble and torque to specs, then repeat five more times.

If you have a heavily damaged journal your new bearings won't last very long.

Click to expand...

:worship: Thanks, this looks like something I could do. However, I may not have to go this route. I just talked to the shop where my Jeep is now parked (the service manager is a friend of mine) and found out that they never even put a wrench to the flexplate bolts before making the determination that the rod bearings were the source of the knock. I'm bringing it home tomorrow, before these guys cost me any money. I was looking for an educated opinion from them before tearing into this myself. Well, I got what I paid for.

The information I have received so far from this forum on the other hand, has been very valuable to me, and I will now get my own hands dirty, in search of the offending knock. Thanks again, I will report back with my findings and any additional questions.

John

sorry for bumping an old thread. I just wanted to add my experience yesterday. although it may not apply 90% of you as I have a 2.8 v6. but I started to develop a knock around the bellhouse. I took off the inspection plate , retightend the tc bolts, and then i noticed the starter was hitting the flexplate. a little round piece on the end of the bendix was broke off, and was hitting the ring gear and spinning on the shaft. replaced the starter. noise gone.:thumbup: