1990 Maxima 165,000 miles.

I just bought this car a couple months ago. The guy I bought it from didnt no much about engine service history as he did not drive it very long due to broken tranny. I had the tranny rebuilt and its seemed to run decent untill lately. I decided to run a compression check on it.

cylinder#/maxPSI

1. 155
2. 180
3. 170
4. 190
5. 158
6. 190

As you can tell the rear head (1,3,5) is noticibly lower than the front.
What kinda numbers are decent? What does a new motor put out? Would re-torquing the rear head help? I dont know much about nissan motors and would appreciate any feedback. Thanks!!

 

maybe a new rear head gasket would fix it. that is rather wierd to have the whole rear bank low and the whole front bank super high.

 

i agree however it could be more than just a head gasket. theres more involved here like the rings n such.

 

valves also... I think I will try a head gasket and see what happens. Also I have access to some in-expensive low mileage NA- 300ZX heads. I believe I heard these will fit on the maxima motor and could provide a performance gain? Is this true?

 

I don't think it's a head gasket, but just my opinion. Usually, you'd have 2 adjacent cylinders with bad compression if the gasket was bad. In this case, the 2 with the worst compression on that bank have one with better compression inbetween....any exhaust manifold leaks?? You also might try a "wet" compression test as well, and see if the readings improve. That might give you an indication of the condition of the rings on those cylinders. Also check the plugs on 1 and 5, and see if they are fouled with oil.

BTW, where are you in Dallas?

 

for a VG motor, the 1,3,5 cylinders are actually about normal. the front ones are very high. that suggests carbon deposits on the 2,4,6 cylinders.
I would first suggest cleaning the engine using some seafoam type cleaner...

another way to clean it out is to disconnect one of the vacuum hoses on the manifold with the engine cold but running and stick it in a bottle of tranny fluid... the tranny fluid has a lot of detergents that will help break up that crap and burn it out of the engine.

you can also do it with water, but it's not quite as effective.

try giving the engine a nice steam/smoke cleaning and then do another compression test after that.

you can also try some injector cleaners, but I'd be worried of what it will do to the injectors themselves. that stuff is extremely harsh chemically, and I'm not too impressed at what they do to the brass in the injectors.

 

fsm states that the vg's compression pressure should be standard of 173psi and a minimum of 128psi, difference limit betweeen cyliinders is 14psi.

if compression in one or more cylinders is low pour a small amount of engin oil into cylinders through spark plug holes and reset compression.

if adding oil helps compression piston rings may be worn, valve may be sticking or seating improperly, or if compression in any 2 adjacent cylinders is low and if adding oil does not help compression, there is a leakage past the gasket surface, if so replace cylinder head gasket!

taken from FSM

 

*ahem* read last post. problem here isn't low compression, it's high. carbon buildup from running rich, most likely.

 

I just wanted to sound smart for once

 

Yep, Matt is right (like he needs confirmation from me..) I didn't look at the book for the specs before I posted. Still seems wierd that there is that much difference front to back. Is it normal to only get carbon on one bank, and not the other?

 

On the VG it sorta makes sense. if you look at the upper intake manifold
http://www.mattblehm.com/pics/old_ma...ne_front_1.jpg

You'll see some hoses on the intake collector just in front of the throttle body... those are the most common places for oil to come into the engine, since they're vents from the valve covers. it's easy for one side to go straight into one port on the throttle body, which the air from that side pretty much stays on one side of the engine.

The PCV valve is baiscally the same way. it actually vents to between the intake runners on the manifold, but if one side gets clogged up somehow, the oil from it will all go to the other side.
http://www.mattblehm.com/pics/old_ma...manifold_1.jpg
(it's the port on the top left corner of the pic and you can see how it runs between the intake runners. there's a small opening at the beginning of each end to vent the gunk out. they're small can can be easily clogged.

 

thanks for all the ideas guys! I actually live in Garland off 78! Small world huh?

 

Makes sense, Matt- thanks. I had my intake apart not too long ago, and I remember mine was gunked just like your pic, and in fact, my highest compression readings were on the front bank- 180-190. Hey- CSID- we're practically neighbors!