drwhosc

So as part of working up my rough running while cold, I decided to check the compression of the engine.

I pretty much got 150 all around except of one cylinder that gave me a reading of 90.

So low compression all around. I did read that I should put oil in the cylinder to get a more accurate reading, but seems counterintuitive to me, but a good bit about the 928 is counterintuitive. Also I am worried bout the low compression in one cylinder. I may have a bad or sticky valve....

So what do folks think I need to do next ???

Mrmerlin

how long has it been since this engine war running and making power?
If its been sitting,
then it should be run for a few hundred miles,
add some MMO and techron to the fuel,
do short drives so it can work.

If it has been driven regularly then you may have a burned valve

drwhosc

It has sat since last winter. I did my front end and motor mounts, and then when i tried to get the car back on the road, very rough cold idle. I had some other stuff to work on, so I put the 928 project on hold. So it has sat for about 9 months.

When I drove the car, the brake pedal wen to the floor. I have not worked it up yet but fairly sure my master cylinder went. So I have to decide if I need to pull the engine or not. I did the motor mounts because I had a engine shake, and it remained after the mounts went in, but far less noticeable.

So I am concerned about a bad valve. I also read it is best to pull the motor if you need to work on the heads.

I am thinking about fixing the brake cylinder and driving for a bit, but I do think at some point I ill have to pull the engine, but not sure how bad this will be for the engine in th e long time, I prob will only drive this car about 1-2K miles a year if that...

soontobered84

We did a tech session one time a while back here in DFW for a guy who couldn't seem to get his '87 fixed by 3 different shops. We determined that there were 3 different things that were keeping it from running and that any one of the three would keep it from running. We gave the owner the trouble rundown and decided to temporize the trouble spots so that we could start his car, he could hear it run, and he would know that we had at least some idea what we were talking about. We got his car started and it ran like dog doo.

After talking to him for a bit, we found out that while driving the car, he had thrown a timing belt. He installed a new timing belt and continued driving the car for about a year until it stopped running from the things we found wrong with it. We decided to check compression.

IIRC, it measured 30 lbs on all cylinders but number 8 which was around 190.
In other words, he bent valves on 7 of 8 cylinders and the engine continued to run for a year.

I wouldn't and don't recommend driving the car like this, but it is amazing to me that it ran at all.

In your car, borrow a boroscope to see what's going on in that one cylinder. If the valve is bent, you know what needs to happen, if it's just a piece of carbon keeping the valve from closing all the way, maybe it can be dislodged, and removed.

drwhosc

I have a buddy who has a scope, and I think that i a great idea. The car actually has plenty of power, and does well, but it does seem like it has a wobble as one cylinder is down. And yes I know what needs to happen, and just not looking forward to it...

Thanks for the push.... But also thinking maybe I should sell as a parts car and take the $$ and get a different 928

soontobered84

As Mr.Merlin says, " keep 'em running"!!!

dr bob

Following on John's excellent advice, consider doing at least part of a leakdown test. My Snap-On compression gauge uses short hoses with plug threads on one end, and the gauge itself connects to the other end using a fairly common quick-disconnect fittiing. Perhaps yours does too? I put a small pressure regulator in the air line, and connect it to the hose via that connector. Remove the oil filler cap. Turn the the engine to TDC compression stroke for the cylinder being tested, then slowly increase air pressure to that cylinder until you can hear where the air is escaping. If it's noisy at the air filter/airflow sensor, an intake valve is leaking. From the tailpipe, think exhaust valve. If you hear air at the oil filler, you have a piston ring leaking.

Adding oil to a cylinder while compression testing offers a chance to improve the seal at worn piston rings. It's usually better to check compression with a warm recently-run engine so the bores are at normal size and still lubricated. Compression is always checked with the throttle wide open, fuel and ignition systems disabled. Pull the EZK fuse to do the disabling.

There is a pretty big span of readings available with different compression gauges. On smaller engines especially, the volume of air in the hoses and gauge affects the readings. Better gauges might have a check-valve in the part that screws into the plug hole to minimize the effect. Not an issue so much with the 928 and the larger displacement. You are usually looking for cylinders even relative to each other more than trying to decide if the pressure is 225 or 190 lbs.

Battery needs to be charged so that cranking speed is the same for all testing. Else the last cylinders tested will likely be lower than the first ones.

drwhosc

Thanks DR Bob. Yea my compression gauge can accommodate a compressed air fitting. Always wondered how to do a leak down test. As soontobered84 noted, I think I do have multiple issues, so I will take my time, and prob get some other heads to think about what to do. I live in Upstate SC, and not too many Porsche and even less 928 resources here. So hopefully I can use the knowledge on th board to help out. Luckily like many 928 owners I have a good case of OCD, and I do plan to keep it running. I am all in on the car....

Petza914

Bill & Frank over at ProTech Motorsports in Greenville are pretty well versed on the 928. They've worked on mine and there are almost always a couple there for various reasons. Maybe have them take a look. If you've not been there before, tell them Pete with the black 928 convertible and the silver RUF 911 sent you.

drwhosc

Cool, I knew they worked on Porsches, but did not know they know 928s... When I get the brakes back, I will take it by. thanks for the heads up