Compression Test using impact wrench to turn engine instead of starter?
 

It has been suggested to me that I can perform a compression test by cranking over the engine with a impact wrench on the crank pulley bolt. Is this an acceptable way to turn the engine for compression testing?

Reason for the question: Engine is out of the car, separated from the transmission / starter, and on a engine stand, pretty much stripped down to the long block. I performed a leak down test while the engine was still in the car (with all losses in the 5-7% range), but did not do a compression check. I guess at this point the compression test would simply be to test valve spring effectiveness. My visual inspection of and manual pressure on the valves / springs / retainers revealed nothing of concern (albeit to my relatively untrained eye).

So, impact wrench to turn the engine over? Yes, or no?

Thanks, Gary S.

I don't think this would work, nor would it accomplish anything. You might over tighten the bolt big time or even break it off. As the compression came up the gun would just twist in your hand or turn the motor over too slowly to do any good. If you have good leak down and no broken valve springs (inspect visually) then there is no point.

-Andy

if there is a choice between compression test and leak down test, and you can only have one, then leak down test is the one to have.

And you drained the oil out?

Doesn't sound like a good idea to me. If you really want to check compression, bolt up transmission, flywheel, and starter. Won't take long.

Why is the engine out/stripped? Are you rebuilding it?

This will not work. You'll end up breaking the bolt.

Thanks Andy. Mostly I agree with you. The only point would be to verify my visual inspection.

dad911, thanks for the input. I'm trying to locate the source of a clacking sound (see audio / video link below). I had several 911 owners come listen to the engine and got various opinions ranging from exhaust leak, valve train noise, and rod bearing. In initial attempts to correct, I pulled off the SSI's and did a valve adjustment, checking for broken valve springs as I went along. Put the SSI's back on with new gaskets at the exhaust flanges and the muffler. Changed oil and filter, fired it back up, still clacking. Did the leak down, results as mentioned above. Next pulled individual plug wires at the coil sequentially and listened for changes. The clacking sound was still evident when the plug wire was pulled for 1, 2, 4, 5 and 6. However, it quieted noticeably when the plug wire for cylinder 3 was pulled. The fellow who suspects a rod bearing cited this information as the basis for his suspicion (no spark to smack a rod with a bad bearing against the crank journal). This seems like the most likely cause to me at this point, but I want to eliminate as many other potential causes as possible before a complete teardown.


Yes, the oil is drained out, other than the residual surface coating. And the plugs are pulled. And the clutch / pressure plate and flywheel have been removed. As well as the CSI system, fan / alternator, shroud and wiring harness. As mentioned earlier, basically down to the long block.

Here is a 45 second video of the engine at slow (warm) idle with the clacking sound clearly audible:

slowest idle video by brg2290 - Photobucket


And lastly, some additional info. The day this clacking sound began, I had already driven the car at city speeds for 20 minutes or so. No unusual noises, and the car was warmed up nicely. I stopped at a stop sign waiting to enter a 60 mph highway. The sound began as soon as I accelerated onto the highway.

Gary S.

a compression test will not tell you if you have a broken valve spring or a bad rod bearing.

you did not say what year your car is, nor how many miles are on it. the 3.2's dont have the best reputaion when it comes to rods.

The car is a '78 SC with the original 3.0. It has 152,000 miles on it. In 2004 broken head studs called for a teardown and subsequent installation of ARP head studs. New main bearings done at the same time, but according to my paperwork, not rod bearings. The work was done by a local P car mechanic with years of experience, including dealership work at Vasek Polak (which doesn't make him infallible, just for reference).

I never intended to use a compression test to check the rod bearing. Knowing that the leak down numbers were all good, why wouldn't a poor compression number indicate the possibility of weak or broken valve spring(s)?

Gary S.

First- I don't believe an impact wrench will work when doing a compression test.

Second - If its a bad valve spring your looking for - forget it. The compression test won't tell you that.

There are a couple of test that could illustrate weak or broken valve springs if the motor were in the car and running. Aside from that - a broken spring could be determined by visual inspection. A weak spring - you need to disassemble and test each spring individually on a spring compression tester.

Good Luck-

I listened to your video and read in further detail about plug number 3. I would expect a connecting rod bearing to have a little higher pitched sound - but having the noise go away upon removing the plug wire is a "classic" symptom of bad con rod bearing - or a bad wrist pin

You can attach a starter to your motor with a starter adapter ring from Zalex Industries.

http://forums.pelicanparts.com/uploa...1292880431.jpg

Although, I agree with the others....a leak down test is better.

Weak springs usually manifest themselves at higher RPM because a weak spring doesn't have enough force to close the valve quickly. Even a proper compression test would not be conclusive because the engine isn't turning fast enough to tell if a valve is a little slow in closing. Besides, springs don't suddenly get weak. The DO suddenly break, but you can tell that with a visual inspection.

Thanks. Based on what I'm reading here, I'm going to nix the compression test altogether.

A wrist pin would seem to be an easier repair to accomplish. Either way, with a clack that loud, I'm thinking I should be able to tell if it's one or the other once the cylinder is pulled.

Nice tool, looks expensive!

Okay, makes sense, and thanks for the explanation wdfifteen.

So, I'll forget about the compression test. Since cylinder #3 was indicated as the likely source of the sound, for now I think I'll focus dis-assembly on that bank.

Again, thanks to all who've posted.
Gary S.

It does sound pretty bad. I don't think it sounds like a rod knock however. I've heard a rod knock and it's a much lower thumping/knocking sound. I do think you'll have to start taking it apart to get to the bottom of it.

-Andy

Hi Andy. Yup, I started getting into it more last night. Basically, just removed the chain tensioner oil lines and cover plate on the left side of the engine, then had to send out an email locally to locate the 46mm crowfoot wrench and holding tool for the cam nut. One of the local PCA fellows has them, so I won't be stuck for too long as I can pick them up today.

Gary S.

Update - Spring 2012
 

So, over a year later, here's an update. This may come in handy for others searching for solutions to clacking noises. Short story - disassembled the engine. Paid a local Porsche mechanic (Richard Buechler - Alpine Motors) to come to my garage and look over the various parts of the engine to see if he could pinpoint the source of the knock heard on the video. Nothing definitive was noted. Continued with an extensive refurbishment of the engine, including: micro-polish crank, re-size rods, rebuilt heads with new valve guides, new main, rod and intermediate shaft bearings sent to Calico Coatings for treatment, lots of new parts from Pelican and clean, clean, clean as per Wayne's book.

Richard did the reassembly for me, and one day during the reassembly I received a phone call from him. He said, "I think I found the source of your engine noise, and you're not going to be happy." I asked what he thought it was. He said that, while rotating the engine on the stand at just the right angle during assembly, he saw an open air injection tap below one of the exhaust ports. Back in 2004 I had eliminated the OEM cast iron exhaust and air injection lines by back dating the exhaust to SSI's. I was told that the mechanic who did the work apparently used NPT plugs and Loctite to plug the air injection ports, even thought the ports are metric threads. One of the plugs had blown out, and this is where the clacking sound had been coming from! In hindsight, my engine did not need to be disassembled. :o

But at the time of this discovery we were well on the way to having it all back together. This was January, 2012. Here are some pictures of the engine.

Gary S.

http://forums.pelicanparts.com/uploa...1334546012.jpg

http://forums.pelicanparts.com/uploa...1334546040.jpg

http://forums.pelicanparts.com/uploa...1334546069.jpg

At least he did nice work. Looks gorgeous.

Gary,

Beautiful engine - if that's any consolation. When I replaced my air injectors, the experts suggested NPT plugs. But I used the correct metric thread ones which can be found for 914 brakes:

Pelican Parts - Product Information: PEL-BRC-02

I know it's disappointing, but it was most likely dirty or oily threads that kept the thread locker from working. I'll double check my plugs next time I'm under the car.