Compression question...
AudiWorld Super User
Joined: Mar 2000
Posts: 13,200
Likes: 2
good point. I never comp tested a dbw car. On cabled cars, there's a hole somewhere for air to
sneak in so, most of the time, depressing the accelerator doesn't improve it much - but the reading just rises up faster on the 1st few cranks but levels off to the same level. I usually crank about 8 times
AudiWorld Super User
Joined: Mar 2000
Posts: 13,200
Likes: 2
buy compression tester from Auto parts store or Sears, take off spark plugs...
which of course means take off coils 1st. screw in comp tester in spark plug hole being careful not to drop any debri in the other open plug holes. get inside cockpit and crank away. Record reading, bleed off trapped pressure and repeat on next cylinder.
Elder Member
Joined: Sep 2002
Posts: 14,366
Likes: 1
On a non dbw you want to hold the throttle at 100%.
On a dbw the thottle plate does not work so you can take it off by just removing the 4 bolts that hold it to the manifold. I did not do that but I was only looking to see if all 4 cylinders matched. Not like all 4 are going to go bad at the same time unless a head gasket goes out.
The main cylinder to keep an eye out for is #4, it is usually the first one to go.
The main cylinder to keep an eye out for is #4, it is usually the first one to go.
AudiWorld Super User
Joined: Mar 2000
Posts: 13,200
Likes: 2
You don't have too. Remember, the cylinders are moving about 200rpm....
Air will enter fast enough through the throttle butterfly not sealed 100%, the idle control valve, the PCV, etc. One more thing, if all plugs are removed, and the cam has an overlap (as all stock do), then air enters through the open plug holes into the slighlty open intake valves to fill the manifold
Remember, that a car idles at around 800rpm and gets close to 100% cylinder filling at idle without any foot on the pedal. That's why if you don't open a throttle while comp testing, you just have to crank a few more times - like 1-2 to get max pressure.
Try it, you'll see.
On a dbw, just pull out a large vacuum hose. No need to take off the throttle plate - but of course, I have yet to do a comp test on one.
BTW, I have 2 comp testers, one is about 16 years old since that's how long I've been using it - in the shop over hundreds of cars. Works good as it's a MAC and I never drop it
Remember, that a car idles at around 800rpm and gets close to 100% cylinder filling at idle without any foot on the pedal. That's why if you don't open a throttle while comp testing, you just have to crank a few more times - like 1-2 to get max pressure.
Try it, you'll see.
On a dbw, just pull out a large vacuum hose. No need to take off the throttle plate - but of course, I have yet to do a comp test on one.
BTW, I have 2 comp testers, one is about 16 years old since that's how long I've been using it - in the shop over hundreds of cars. Works good as it's a MAC and I never drop it
Elder Member
Joined: Sep 2002
Posts: 14,366
Likes: 1
When I did my stock motor while I had the compression problem I got
a low of 120 and the highest was 165 which meant I did have a few cylinders down. Now with the new 8:1 compression motor I show a 135 psi in all 4 cylinders. I think the stock 9.5:1 was around 170-180 psi. Only problem is that I have never checked a good stock motor, but I will have a chance to check a 2001 9.3:1 motor tonight.
AudiWorld Stories
Bringing Audi to Life for Audi Fans
2027 Audi Q7 and SQ7: Audi Upgraded EVERYTHING!
Michael S. Palmer
Audi Unveils Absurdly Cool New Supercar: 10 Things You Need to Know!
Verdad Gallardo
The Highs & Lows of Every Audi C-Class Generation
Joe Kucinski
Top 10 Most Expensive Audis Ever Sold on Bring-A-Trailer
Brett Foote
10 Audi Features & Options We Miss the Most!
Joe Kucinski
Audi Recreates Crazy-Looking Speed Record Breaker From 1935
Verdad Gallardo
Coachbuilder Recreates the 1995 Audi TTS Concept
Verdad Gallardo
Every Audi V10 Car Ranked!
Joe Kucinski
9 Audi Designs That Aged Like Fine Wine
Verdad Gallardo
Elder Member
Joined: Sep 2002
Posts: 14,366
Likes: 1
But your motor has 9:1 compression and I just tested a stock 9.3:1
2001 motor with 20K miles on it and it still put out 185 psi for every cylinder. I will do the exact same test on my 8:1 and see what I come out with since it has about 5K miles on it now.
Thread Starter
AudiWorld Senior Member
Joined: Feb 2001
Posts: 900
Likes: 0
Re: Compression question...
I'm not sure to be honest, the mechanic gave me this sequence 140,140,140,135. I'm gonna find out. I also noticed with the sluggish sensation that my boost gauge reads a little different. At cruising speed the Vac reads at 2-5 and not 10 as before (psi). I'm pretty sure this is related to the sluggish sensation. It is very hard to explain but something is not right.
Mau
Mau