What's the procedure? I just need to check the compression on one cylinder since that's the only one that seems to run lean on. Here are the pics of spark plug from cylinder number 4.

http://public.fotki.com/1max2nv/1max2nv/engine/

 

pull all plugs, screw in compression tester, open the throttle all the way and turn the engine over 3-5 times

 

Man I hope the tip of the plug broke off AFTER you removed it..

and it's white..


Just disconnect EVERY coil pack, Plug the tested into the cylinder you wanna test and crank it 7 TIMES.

Then read the result and your done

 

Do you need to open the throttle ?

I have not heard that and that is not the way I have been doing it.. man I hope I have been doing it correct

 

i read somwhere that you could get a lower reading with the throttle closed

 

I heard that you should open the throttle for a better reading as well...

wow... did that tip break off in the engine?

 

I pulled a plug out of a 91 300zx TT my buddy bought with only 26k miles on it.

The guy who owned it before had the turbos turned up from the stock 9PSI to 18PSI with no other modifications except exhaust.

Strangely the engine survived the abuse and after replacing the plug runs fine. The Turbos are blowing out oil into the exhaust because the seals are blown however.....he's doing an Turbo rebuild now.....

(Moral of the Story - sometimes a good deal isn't really a good deal)

IanS

 

It didn't break off. It melted off. I don't know how I ran lean in just that one cylinder. It might have happened previously from a nitrous run I did. I'm going to double check the fuel delievery to that one cylinder and see if I can pinpoint the cause.

 

HOLY SHIET!


Which clyinder BTW?

 

#4 from the first post...

 

Number 4. Anyway, I replaced the spark plug tonight but I didn't drive the car yet. I'll do the compression test tomorrow after I go buy the gauge.

 

make sure the engine is at operating temperature before doing compression test, if not the results won't be accurate.

Nick.

 

FWIW, I've gotten the same readings with a warm engine, cold engine, open throttle and closed throttle.

I don't think it makes a whole lot of difference.

 

I've gotten slightly higher readings with the throttle open, and engine temp didn't seem to make any difference.

You really want to check all your cylinders, since variation between cylinders is important, too.

My last check I did I got 197 low and 205 high, the other four were right at 200. A little high, but not bad for 140,000 miles.

 

If I do get good compression reading, would it be worth it to get a leak down test done by a shop? I know a leak down test can pin point where the cylinder is loosing compression but what test can I do to see if my piston surface is still good.

 

If you are looking for detonation damage, then the only thing I can think of (other than a teardown) is to use a borescope (also known as a boroscope) to look at the piston surface.

http://www.tooldesk.com/shop/borescope1.wml

But if your compression is good, why worry about?

 

Warm the car to normal temp, leave it running at idle, pull the fuel pump fuse while its running to get all fuel pressure out of the fuel system (very important to prevent wash down which will result in low readings), pull all plugs, throttle open, crank till the reading stops climbing, 10-15 for me. What I discribed is the short version of the FSM procedure, thats how I do it.

 

this is the only way to do it right !

Nick

 

I don't want to sound lazy...okay...I am. Do I have to pull all the plugs? Can I just pull the one I want to test and just disconnect the coil packs for the rest?

 

Yes, you can do that. But if you get a number that is marginal, like 160 psi or so, you're going to wonder if that is due to damage to that particular cylinder or are all your cylinders low? If you get 185 or above I guess you can stop.

But really, it only takes about 30 minutes to do them all, and you should be monitoring cylinder compression and spark plugs pretty frequently anyway to keep track of the overall health of your engine.

 

That's true...I'll just do all of them. I put some 1 step colder plugs in there last night but I should change them out for 2 step colder while I'm at it. Thanx of the advice everyone and I hope that one cylinder survived.

 

http://www.cybrrpartspro.com/Chilton...61CH03_26.HTML

 

Looks like I did some damage to cylinder number 4.

205 205 205
190 180 190

I found out why it ran lean. It was the short nitrous test run I did. There was a small spec of something that blocked the fuel jet for cylinder number 4. I was getting at the most 50% fuel to that cylinder due to the blockage.

 

I think you're OK man - that's not a horrible reading.

Still well within spec.

IanS

 

Yeah, it's definitely not worth doing an engine swap over. Unless you want to, of course.

 

I learned a hard lesson. I'm going to start chaning my fuel filter every 20K miles. I don't believe I have a fuel solenoid filter so I'm going to add one.

 

I bet if you put some mile on the motor the reading will pick up a bit. Wear back in, if thats possible