Driving with bent valves

acura
integra

#1

Dear Car Talk:

I have a 2001 Acura Integra LS Coupe. 4 years ago, my timing belt broke. I wasn’t aware that replacing the timing belt was a preventative maintenance issue and the belt broke on me just as I hit 105,000 miles. A family friend who is a mechanic replaced it for me along with the water pump and all the usual parts that go with a timing belt job. About a year later, I started experiencing low RPMs at idle, slight shakiness, and even once or twice, the car turned off after shaking/low RPMs right after turning over the engine. I went to several mechanics over the next 2 years, going through several attempts to correct this issue (2 tune-ups, throttle body cleaning, fuel line flush). The Check Engine Light diagnostic indicated random misfires, (which eventually led to my catalytic converter needing to be replaced). One mechanic (a Honda specialist) eventually did a valve adjustment and fixed the problem! However, a little less than a year later, the problem came back: extremely low RPMs at red lights/when idling. I grew used to putting my car in neutral and revving the engine to keep it from shaking. Maybe you’ve already guessed what the issue is, but after taking it back to the Honda expert, he found that the car needed a valve/head job. After diving further in, he ended up having to replace 6 valves because several were bent. (I NOW KNOW I HAVE AN INTERFERENCE ENGINE). From everything I’ve read about being “unlucky” and having a broken timing belt on an interference engine, I’m wondering how my car was able to last so long with bent valves. Am I lucky in my unluckiness? I wasn’t driving very fast when the belt broke but it’s amazing that the car lasted this long while I keep reading that a broken timing belt on an interference engine results in catastrophic damage.


#2

Well, it was catastrophic damage. You just happened to delay dealing with the consequences. :wink:

How much damage happens when a timing belt breaks depends on a lot of factors, including RPM, the exact configuration of the pistons/valves when the belt snaps, etc.

It’s kind of like going into a casino and playing Roulette. You have no idea where that ball is going to land, but odds are it’s gonna land somewhere that will cost you money.

The reason it’s usually considered catastrophic is that many people choose not to repair t-belt damage - they either replace the engine, or the entire car, both of which are very expensive propositions.


#3

If one or more exhaust valves get dinged but the resultant leak is so minor that the engine runs with no noticeable problem the mechanic would assume that all was OK and congratulate the owner on their good luck. But then sooner , if not later, a few seconds of hard acceleration will burn one of the damaged valves and result in a leak that causes multiple cylinder missfire.

But that’s just idle speculation from out here in no where land. I have pulled the heads off quite a few interference engines and always found slight dings in one or more pistons usually made by exhaust valves and if tested for leaks all the exhaust valves would indicate seepage. The damage usually goes against logic as I always thought that a broken or stripped timing belt would result in the cam suddenly stopping and whichever valves were at or near fully extended would be totally trashed. But so far I have only seen one engine with a valve stuck in the top of a piston resulting from a broken timing belt. So much for red neck logic.


#4

Extremely.


#5

If you want to keep it, you need a top end job on both cylinder heads. While the heads are off, the pistons and cylinder side walls should be inspected for damage.


#6

The scary part of this story is that none of the mechanics after the broken belt bothered to do a compression test or a leakdown test to determine if any of the valves were bent. I am not a professional mechanic and that is the first thing I’d do after installing a new timing belt if one broke.


#7

Great post. I couldn’t wait to hear what the mechanics thought about this one.


#8

I agree.


#9

A valve lash check can also spot bent valves.


#10

Interesting. What do you look for during the valve lash check - clearances got greater or smaller? Some pattern to the off-spec gaps? Can any damage be seen, or do you conclude damage from the off-spec clearances?


#11

Lash gets larger, since the valve can’t move to its normal closed position.
The one time I encountered an engine with bent valves (a friends '81 Accord)
the lash was so open on a couple of the valves you didn’t need a feeler gauge to tell the rocker arm had far too much free play.


#12

I see. But then what would explain the symptoms not showing up for a year after the broken belt? Maybe the friend who put in the new belt, etc. also adjusted the valves…


#13

@Rod_Knox already explained that - the valves get burned due to being bent. Over time, this makes the symptoms worse.


#14

Learn something every day. I had always thought that it was a yes/no situation, either the valves weren’t bent or the engine was toast. Now it’s obvious that there’s a big middle ground, with enough damage to eventually kill the engine, but allow it to be driven for quite a while.


#15

Wouldn’t bent valves show up in engine performance right away, because they wouldn’t be opening and/or closing fully and, being bent, would not seal on their seats?

Maybe adjusting clearances along with the T belt job brought performance back to OK, and the unevenly closing valves eventually burnt, or damaged their seats, so the loss of compression became much more noticeable.


#16

I’d think with a very minor bend the engine would run but not score well on a leakdown test. Or the valve clearance check. (I like that idea on solid lifter engines!)

The leakdown would get worse and worse as the valve and the seat burn from not sealing until the engine would barely run.


#17

You might be surprised by how an engine will run with valves way out of adjustment. My college roommate had a Capri with the ‘Pinto’ OHC 4 in it. Made a HUGE valve racket, but he still drove it daily. Popped the valve cover, and a crescent-shaped divot was worn in each of the cam followers, and the cam lobes were also worn. I tried adjusting it, but there wasn’t much that could be done, besides replacing all that. He just kept driving it.


#18

I was thinking about that after reading part about “how it ran so go for as long as it did”. I suspect that a few valves were only very slightly bent and so when the combustion occurred it allowed a lot more heat to get to the stem right at the base of the valve head than it would typically get if it had not had any leakage at all. I would guess some valves might actually straighten a bit while others may have bent more or as another person who replied mentioned, it eventually caused a burn valve situation. I grenaded an engine at the dragstrip one time…actually I’ve grenaded quite a number of engines at the track but this one in particular, what happened was one valve spring retainer split allowing the valve to drop into the cylinder. You’d be amazed how much damage can occur, when that happened. The piston literally desinigrated. Of all the shrapnel I gathered up during the autopsy, the largest piece of aluminum was only about the size of a dime. It not only destroyed the piston, it shattered the cylinder wall. Then, what was left of the rod that was still swinging for a couple of seconds busted through the opposite side of the block and took a brand new Mopar Performance mini started out…snapped it right off at the mount. When the piston exploded, that intake valve must have been open or it was the one that dropped, it’s been a few years, and all the debris shot through the intake and bent almost every valve in the engine. When I built the engine, it was one my first 500" stroker motor, I didn’t know about needing to use titanium valve spring retainers. The titanium isn’t any tougher I think they don’t split under extreme high rpm and ridiculous amounts of valve lift, duration and overlap because they are lighter. Steel retainers are actually stronger but not better. I saved one of the pistons that didn’t receive too much punishment and made and ash tray out of it. I call it my 10 thousand dollar ashtray. There was some damage to that combustion chamber that had to be repaired, the crank was junk, the even the camshaft twisted. A ten dollar part would have saved me all that money. I always do the research on a motor to find out if it’s an interference engine, has a timing BELT, etc. There are a lot of cars out there that I wouldn’t risk even going as long as the recommendation to change the belt. It’s not a cheap job but the damage it cases when it goes…It’s well worth the cost of the belt change in my opinion.

Good luck!

Wayne


#19

Valves can strike the piston and not bend the stem at all but damage the head. The sealing surface of the valve is only a few few millimeters wide and the margin below the sealing circumference can become damaged resulting in a hot spot that eventually burns.