Compression Test

[HB280ZT]

HI All,

 

Well my 82 turbo motor is not running up to par, it has a miss or a hesitation, not sure which it is but the car just stumbles. So today I checked the plugs, cap and rotor, the wires and did a compression test to see how things were and here is what I found:

 

Plugs all looked good.

The cap and rotor was good also.

The wires checked out.

 

Compression test.

#1 - 118 and 115 psi

#2 - 115 and 115 psi

#3 - 115 and 115 psi

#4 - 118 and 115 psi

#5 - 115 and 115 psi

#6 - 115 and 115 psi

 

I understand that max pressure is 142 psi and min is 100 psi and that all cylinders are within 10%.

 

Just as a reference the motor only has 35,000 miles on it!!

 

So what to do???

 

HB280ZT

[technicalninja]

Compression is fine, All comp gauges vary somewhat and one gauge might show 150 where another would show 165 and yet another might be at 125. I have verified this strange fact by testing all the gauges in a dealership (more than 10 mechanics/gauge sets in test). Even had large variations between gauges made by one source (Snap-On, not a cheap source). What I look for is readings close together. Your engine has a smaller variation factor than most engines, even freshly rebuilt stuff. Doesn't appear you have engine or ignition system issues but ignition patterns should be looked at on a scope to verify ignition is actually working properly as some problems cannot be diagnosed by part inspection.

I would look to an injector/injector pattern problem next. Also might be vac leak issue.

[Guest bastaad525]

yeah, had that happen to me too. I bought some gauge from autozone for cheap, and it was showing really low compression on my rebuilt L28ET, like 110-120psi, then I used my friends guage which I think was a snap-on brand, and it was up at like 140psi. I mostly just look to see that they are all within close limits of each other, across all six cylinders. If a motor is very low compression i.e. worn out, it will let you know with general low performance overall, high oil consumption, possible smoking, excessive valve train noise.

 

Problems like misfire and hesitation are almost always problems with the EFI, either corroded connections or a bad or misadjusted sensor.

[HB280ZT]

Hi All,

 

Well I am bringing back this thread because I just did another compression test of my tired motor. Not to mention that I replaced the P90 head with a N42 head last year and blew a head gasket 2 months ago. Because I changed the engines compression ratio from 7.2 - 1 to 8.3 - 1 the compression test should have read higher with this setup over the original setup.

 

What prompted me to do this was oil being sucked into the intake runner even though I have a catch can in place.

 

Here are the results:

 

1 - 125/122

2 - 120/122

3 - 110/110

4 - 115/113

5 - 100/97 wet 135

6 - 110/110 wet 165

 

Now I have been having fun with the car at VIR over the last few months and I know the power is done some from when I did a dyno test 8 months ago, i.e. 250 now vice 300 then. I know some dynos will show different but 50 rwhp is a lot of change.

 

So what do you all think? Do I need a new motor or what?

 

HB280ZT

[Mayolives]

Harry,

It's time to spend some of the rusty Coast G. money and freshen up the motor.

[HB280ZT]

Spend some of the rusty Coast G. money?? What Coast Guard money I was in the Navy!!

 

Yea I was thinking the motor was on it way out but I had lots of fun with it at the track anyways. So now I will need to work on building a new motor. So what do I really need????? Will have to see how money is after a move and starting a new job.

 

So for the people in the Tampa area is there someone down there that knows how to work on Z motors??

 

HB280ZT