Unusually high compression!?!

[motor-ray]

I have a 1982 280zx n/a with an automatic transmission. My end goal is to make it a nice driver and do some lapping days with it later on once I do a 5spd swap. Before I do anything major I want to get the car in good running condition and here lies my problem. The car has a lot of hesitation off of idle and will sometimes backfire through the intake. It has also become hard to start when its hot. I have done the following things to the car thus far: new spark plug wires, new spark plugs, distributor cap/rotor, fuel injection pig tails, egr, dist pig tail, air by-pass valve pig tail, st timing per FSM at 8 degrees btdc, new K&N factory style air filter, new fuel filter, replaced several rubber efi lines.

 

When I recently did a compression test I got a range of 205-225 psi with cylinder 6 being the lowest but never lower than 205. This seems exceedingly high for what appears to be a stock engine with a P79 head. Could this be the source of my problem? Has anyone else encountered something like this? Am I an idiot and did the test wrong? When I tested it (with a good quality Matco gauge) I took the air cleaner lid off, removed all the spark plugs, and held the throttle wide open. The car does seem to run better if I set the ignition timing closer to 15 degrees BTDC which is far as I can adjust it. But, it is still not right. I have excellent spark and I believe my fuel injectors are ok (put a screwdriver against each one and they sound like they are all firing). But, the car does appear to be running rich at idle (smells rich but plugs are not exceedingly carbon fouled).

 

Hopefully this hasn't all been covered before but I did not find anything searching the forums. Any help would be greatly appreciated!

[SHADY280]

i wouldnt be concerned. it just means that someone may have raised up the compression ratio. if so you should run higher octane fuel. did you put a bit of oil in the cylinders to do a wet comp test? this even raises the numbers more. that being said, it could mean the valves are getting stuck a bit, but i highly doubt it as all the cylinders are close to the same. running rich means the person that upped the cr may have played with the afm to try and get more fuel for the top rpm range. 15 deg is fine, as long as no detonation is heard under hard load etc. with the cr upped, youll have a nice snappy motor with more power than stock. enjoy

[motor-ray]

i wouldnt be concerned. it just means that someone may have raised up the compression ratio. if so you should run higher octane fuel. did you put a bit of oil in the cylinders to do a wet comp test? this even raises the numbers more. that being said, it could mean the valves are getting stuck a bit, but i highly doubt it as all the cylinders are close to the same. running rich means the person that upped the cr may have played with the afm to try and get more fuel for the top rpm range. 15 deg is fine, as long as no detonation is heard under hard load etc. with the cr upped, youll have a nice snappy motor with more power than stock. enjoy

 

Thanks for the response. I did not do a wet test. So if the compression is not the issue, do you have any ideas for the hard starting when hot, hesitation under light acceleration? I know the AFM has been monkeyed with as the mixture screw is visible (I believe they were capped from the factory?). Does anyone know the baseline settings? Are there other likely culprits? Thanks again!

[johnc]

Download the factory service manual and go through the basic tune-up procedures. Also, get this book and go through the diagnostic procedures.

 

http://www.amazon.com/Bosch-Fuel-Injection-Engine-Management/dp/0837603005/ref=sr_1_1?ie=UTF8&qid=1292169880&sr=8-1

[BLOZ UP]

Are you sure you still have catalyst? Or did the PO punch it out?

[aarang]

Just because you have a "good quality Matco gauge" does not make it correct. Try a different gauge and see what happens.

[motor-ray]

Download the factory service manual and go through the basic tune-up procedures. Also, get this book and go through the diagnostic procedures.

 

http://www.amazon.com/Bosch-Fuel-Injection-Engine-Management/dp/0837603005/ref=sr_1_1?ie=UTF8&qid=1292169880&sr=8-1

 

 

Thanks for the tip on the book. It looks like a good one! I have the FSM and the original fuel injection book and have been reviewing it. Is there anyway to truly verify if the injectors are clean and operating properly? I have never dealt with EFI and am learning as I go.

[motor-ray]

Just because you have a "good quality Matco gauge" does not make it correct. Try a different gauge and see what happens.

 

You have a good point but I also do not have a second gauge to check against. So I hooked my gauge up to my air compressor with the regulator set at 85 psi. My gauge read just under 100 psi. It does explain some of the problem but not all of it. I guess I need to find a Matco dealer and see if my gauge can be recalibrated. Thanks for the tip.

[motor-ray]

Are you sure you still have catalyst? Or did the PO punch it out?

 

I am not sure if the catalytic converter is still there. My guess is that it is gone (I'll have to jack up the car and take a peek)as they had made their own y pipe to go to the exhaust. Will the lack of a catalytic converter cause any problems?

[motor-ray]

Are you sure you still have catalyst? Or did the PO punch it out?

 

I tried to check out your website but nothing came up on the home page. Still a work in progress? I would like to check it out when you get it going. On another side note, this 280zx came from DFW (northern Grand Prairie).

[mario_82_ZXT]

Possibly high carbon buildup causing an increase in compression?

[JMortensen]

Possibly high carbon buildup causing an increase in compression?

Seen that before in older Volvo engines. They complain about pinging, turns out they've got 1/8" of carbon on the pistons.

[BLOZ UP]

I am not sure if the catalytic converter is still there. My guess is that it is gone (I'll have to jack up the car and take a peek)as they had made their own y pipe to go to the exhaust. Will the lack of a catalytic converter cause any problems?

 

Your car will smell more like gas/death than normal.

[SHADY280]

wow that guage is off. or your compressor guage is off. there not always accurate either, but a good start to something fishy. sounds like you need a wide band o2 to see what the mixture is. thats the way to somewhat calibrate the afm. with these cars being monkeyed with so much by po's you never know what you have. even what looks original may not be, plus age has to play a role as well. id run some seafoam thru it to reduce the carbon possiblity, you can use water as well, but its whatever floats your boat. then you can play with timing and mixture.

[BluDestiny]

I'm amazed no one asked this yet, but what is teh history on the car? Did the PO tell you anything he did specifically to it, like machine work etc?

[motor-ray]

I'm amazed no one asked this yet, but what is teh history on the car? Did the PO tell you anything he did specifically to it, like machine work etc?

 

No real history on the car. The last owner was a guy with no mechanical skills that paid to have it worked on when it broke, which is why he got rid of it. I do not believe he had any major work done to the car while he owned it (judging by the receipts he gave me). My guess is any major work must have been done prior to him (if there ever was any).

 

Also, the more i think about this it may be all in the gauge as it appears to have a 15-20% error in it. At 200lbs the true reading would be close to 40lbs off which would put me close to factory. A little carbon build up could make up the difference.

 

Someone mentioned a wide band O2 sensor. Is this similar to pipe sniffer (probe in the end of the tail pipe) set up or is it something different?

 

Could it be my cam timing is off by a tooth? I know this is easy to do in some engines and would cause similar issues in these motors.

[SHADY280]

if your cam was off, you would know it. unless the po installed it wrong, these things typically dont jump, they have a double roller chain. the o2 sensor goes in a welded bung in the exhaust pipe close to the engine. it reads digitally and accuratly the air fuel ratio so you can tune the engine. you need the wide band type. the narrow band type is only good for full throttle, and isnt that great. I never trust what a po did anyway, they usually lie to sell the car, even with recipets

[BLOZ UP]

Someone mentioned a wide band O2 sensor. Is this similar to pipe sniffer (probe in the end of the tail pipe) set up or is it something different?

 

 

Yes, but they are faster reacting since they are further up the stream.

 

the narrow band type is only good for full throttle

 

The narrow-bands are usually only for cruising, when the car is warmed up and in closed loop mode. Any high-load situation and car reverts to whatever its fuel map says with no adjustments. At least that's how it worked for most of the cars I worked on.