Effects of Heads with compression difference = 35 psi? - 3000GT/Stealth International Message Center

lockeal · 05-06-2008, 12:04 PM

I had a cracked head on my 91 DOHC so I bought a rebuilt one and had the other one rebuilt by a local shop. The car has a hesitation/miss under low load for the first 15-20 minutes of driving and then is fine.

My compression #'s in PSI:

Cylinder Warm Engine With oil added
1 175 190
2 150 185
3 170 ---
4 155 200
5 180 ---
6 145 190

All are above the min 139 psi in the manual. However, the 35 psi compression difference from high to low is well above the 14 psi limit in the book. It doesn't say what will happen if there is this large difference like this? Will this cause the hesitation when cold?

As the low compression increases with the addition of the oil, it indicates bad rings/piston but they looked fine with cross hatch and everything.

Thoughts?

lockeal · 05-06-2008, 05:11 PM

The first number for each cylinder was with the engine warm. The second number is with a litle oil added.

I just checked the compression with the engine now cold and the range on the back cylinders vary from 145 to 155 but the front cylinders only go from 150-160 now. I had checked the front with the plenum but opening the throttle body. I checked the compression on the back cylinders without the plenum of course so maybe that was why the the front ones were higher...

Does this mean I'm OK and should look elsewhere for my cold hesitation problem?

05-06-2008, 12:04 PM	#1 (permalink)
lockeal lockeal Join Date: Oct 2006 Location: State College, PA Drives: 91 3000GT SL Trader Rating: (1)	Effects of Heads with compression difference = 35 psi? I had a cracked head on my 91 DOHC so I bought a rebuilt one and had the other one rebuilt by a local shop. The car has a hesitation/miss under low load for the first 15-20 minutes of driving and then is fine. My compression #'s in PSI: Cylinder Warm Engine With oil added 1 175 190 2 150 185 3 170 --- 4 155 200 5 180 --- 6 145 190 All are above the min 139 psi in the manual. However, the 35 psi compression difference from high to low is well above the 14 psi limit in the book. It doesn't say what will happen if there is this large difference like this? Will this cause the hesitation when cold? As the low compression increases with the addition of the oil, it indicates bad rings/piston but they looked fine with cross hatch and everything. Thoughts?

05-06-2008, 05:11 PM	#2 (permalink)
lockeal lockeal Join Date: Oct 2006 Location: State College, PA Drives: 91 3000GT SL Trader Rating: (1)	Re: Effects of Heads with compression difference = 35 psi? The first number for each cylinder was with the engine warm. The second number is with a litle oil added. I just checked the compression with the engine now cold and the range on the back cylinders vary from 145 to 155 but the front cylinders only go from 150-160 now. I had checked the front with the plenum but opening the throttle body. I checked the compression on the back cylinders without the plenum of course so maybe that was why the the front ones were higher... Does this mean I'm OK and should look elsewhere for my cold hesitation problem?

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