I need some advice. I have a significant (30%) compression leak down on cylinder 4 of my 1987 928 S4. I know I should have had it checked out before buying the car but I didn't and now it's time to pay. Cold starting is sometimes difficult and cold idle is poor. First off I'd like to determine if the problem is related to valves or piston rings. Any suggestions on how to determine which one? I considered using a borescope but a marine mechanic friend of mine said he's used one before and wasn't able to determine what he was seeing. If the heads need to be pulled is it just worth pulling the motor instead to do a complete rebuild? The car has 105k miles on it and is in good shape otherwise. If I pull the motor I was thinking about cleaning up the heads so it would breath better and ceramic coating some parts. ie top of pistons... see website -&gt;<a href="http://www.performancecoatings.com/" target="_blank">PerformanceCoatings</a> I'm also considering trying a decarb service where solution is injected into the airstream while the engine is running. I know its a high compression engine but after 100k+ miles there has to be some carbon buildup. Any advice would be appreciated. By the way this forum is great! I've found answers to questions here I doubt I could have gotten answered elsewhere. Thanks for all the help.

 

A leakd0wn test normally gives instant indication of the location of the compression leak.

If the compressed air leaks from the intake manifold, an intake valve is leaking.
If from the exhaust pipe, and exhaust valve.
If from the oil filler, the rings are leaking.
If from the cooling system pressure cap, a head or head gasket.

Worst case - and most likely - for you might be a bent intake valve caused from a failed timing belt.

The piston rings and cylinder walls give very little trouble on the 928.

 

Thanks Wally,
Could a bent valve result in only a 30% leakdown though? By the way I used to live in Marietta, GA before coming to Florida. Where in Acworth is your shop? (I assume this is where you're at based on your FROM: location) I'd like to look you up when I'm in town.

 

Oops, I see from your website your in NC. Sorry.

 

Mister,
I am positive surprised that Wally said: '... normally no problem with rings and cylinders on 928..' i.e. that's only me who had that problem!
I guess it is broken rings - I had that two times on cylinder # 4 on my 1980 engine - That's the weakest cylinder in a 928 according to my experience.
Why?
Some possibilities:
- Detonation - Oil driven detonation (Devek theory)
- The rear cylinders are always 'hottest'
- Bad knocking system
- Too lean mixture
- The engine has been too hot - Bad sealing in front of the thermostat, bad thermostat, bad
radiator
- Etc.ect.
If it is due to broken rings, most properly the cylinder is scratched, and you can see it with a special telescope.
I am very interested to hear what it is, when you find it.

 

928 Specialists used to be in Kennesaw, until they moved to Spruce Pine. I am in rural West Cobb. I don't have a commercial shop, but would be happy to see you if you are in town.

Yes - a valve can be bent any amount from none to a lot. A slightly bent valve could cause your leakage.

I was aware of Erik's problem - but it is still unusual. It is astonishing how durable the pistons, rings, and cylinders are on the 928. We sell a lot of 928 parts, and a VERY small percentage are internal engine parts.

 

As Wally and others correctly point out, the leakdown test can help determine the location of the leakage, usually by listening for where the leakage noise is comiing from.

I had a recent conversation with an owner who had experienced a similar problem. Low compression and high leakdown in one cylinder, cauong uneven running at idle and low power. Problem was isolated to the intake, and carbon buildup on the valve tulip was the cause. The engine was treated with some aggressive EFI cleaner to clear the carbon.

Over the years there have been more than a few 'treatments' available for removing carbon from intake valves. The selection is limited now that we have to worry about fouling the cats if we use the wrong chemicals. An effective method uses water drawn into the intake manifold through a vacuum port. The water hits the hot valve, and the thermal shock causes the carbon to break away and get drawn through and out the exhaust. Hmmm. This might be something that you want to find a local talent to try with you, if indeed the leakdown shows that the problem is an intake valve. Drawing too much water risks other engine damage, so this isn't something to try haphazardly.

Maybe a megadose of Techron would be a good first attempt.


Just a thought... <img src="graemlins/yltype.gif" border="0" alt="[typing]" />

 

There is a process available that uses walnut shell fragments to clean the back of the intake valves.

Basically, you pull the intake and "sandblast" the ports and valve with shell fragments. The idea is that the stuff is aggresive enough to take the carbon off, but will not do any damage if it gets into the cylinders.

I still have a can or two of an old additive called "The First Step in A Tuneup" - basically butyl cellosolve. GM still sells the stuff as "GM Top Engine Cleaner" - works really well.

If the engine is running reasonably well, I would vote for dr. bob's suggestion - run Techron for a few tanks and see what happens.


Wouldn't hurt to change the oil a couple of times, as well.

 

The car seems to have plenty of power. Of course I don't have another 928 to compare it to. Once it's warm the car runs fine. It doesn't use oil, which I figured it would if had a broken ring. It doesn't use water or boil over which suggests the head gaskets ok. I hope it's just carbon build up hanging up a valve. Is there a good DIY decarbonizing product out there or is this something I should have a local garage with speciality equiment do for me?

 

Interesting thread.

When ordering a resupply of Redline motor oil this morning, I came accross their SI-1 injector and valve detergent. I added a couple of bottles to the order. Then I read this theread. So here's the scoop on the SI-1 as described by Redline:

"A concentrated package of the most powerful high temperature detergents available to clean gasoline fuel injectors and intake valves. SI-1 can clean injectors to nearly 100% efficiency in one treatment. The cleansing effect on injector deposits can raise fuel economy approximately 12% or even greater, depending on the condition of the injectors. Although many gasolines advertise injector cleanliness, GM presented a paper to the SAE which indicates that their customers encountered injector plugging with all major brands of gasoline (SAE 861533). SI-1 also contains detergents and a synthetic upper cylinder lubricant which will help clean the intake valves, pollution control valves, combustion chamber deposits and carburetors. SI-1 also protects the fuel system from rust. One bottle treats 15-100 gallons and provides intake system cleanliness for unlimited mileage. Safe for continual use."

I have always used Techron, but I thought this was worth a try.

Any BTDT on this product?

Greg

 

One last thought--

Going forward, consider making the switch to synthetic oil. Synthetid is much less likely to coke/carbon up the valve stems. If you find that you do have coking on just one valve, it's quite possible that it's due to an intake valve stem seal problem.

Also--

Your mechanic can use the borescope to inspect the valves by pulling the side covers off the intake manifold. Snake the 'scope down the tubes from the opposite side of the engine and you'll be able to see valve deposits up-close-and-personal, especially since there's plenty of room to snake a FO light source down there next to the head of the 'scope. Don't ask me how I know this, just appreciate that I did a --very-- thorough post-purchase inspection.

 

Wally,
I'm going to first try the GM Top Engine Cleaner you mentioned in your post. How exactly did you use it? Did you pour it in slowly or introduce it in vapor form? The local GM dealer said it's available as an aerosol or liquid but all they have and use is in liquid form.

 

I have actually been VERY hesitant to use this on my 928s. I use it on other vehicles, and it works very well - BUT ...

My 928s have the MAF sensor that uses a very fine platinum wire to sense air flow. I don't think that I want to risk pouring a liquid over the wire.

On further reflection, I don't think that I should have even mentioned the product on the 928 list, although it might work really well on the CIS cars.