Low compression plus knocking

[diplomat 1]

First off My fuel gauge doesn't work. I was driving and out of no where the car started to feel like it was about to run outta gas I would step on the pedal and nothing won't move went to the gas station put in about 3 gallons of 93 and everything was fine. The next day I'm driving and it did it again and I found it strange ok how it ran outta gas so quickly but again I went and put in more gas started and ran perfect. I went toy friends house let the car sit for 5 hours came back started back up the rpm was a little low. It was still running smooth tho. I drove off and the same problem started happening again but this time more frequently. All of a sudden as soon as I stop giving it gas it would die I would have to restart everywhere so while the car was moving if I let go of the pedal it would die but if I went into 2nd gear let off the cluTch it would turn on by itself again. I'm on my way home and the car shuts off on me on the highway after a could back fires it does no start. I look under the hood and my tps is hanging off no bolts. I figured that's why my idle problem is but I got a new bolt reset the tps and still no good. I checked for spark and fuel and everything is there my worry is I blew my engine car turns over just doesn't start. I also tried different ecu,ptu and ptu wiring. I also had a code 21 at the time the car died. Funny thing is I been messing wit the car today and it just ran for 2 seconds and died but no code 21 even wit the same ptu I changed the tps set it to .43 and while starting it repeatedly pressing the gas pedal and that's how I got it to start for the 2 seconds. So then I got the car running but still only for seconds at a time and the motor makes big knocking noise and won't rev over 2k rpm. I have oil in the car and like I said the oil pressure gauge in the car reads good pressure. I have a harbor freight compression tester so it sounds like air leaks out of the fitting for the tester so I had someone crank the car while I help the tester. I followed a thread I found on how to do a compression test ( unplugged fuel pump and cas and wide open throttle) here are the results ( since it doesn't hold pressure the reading are as close as possible to eat I saw)
1: 150-160psi
2: 150-160psi
3: 150-160psi
4: 70-90 psi
5: 150-160psi
6: 80-90 psi

I am going by this cylinder identification:

Back
5 6
3 4
1 2
Front

So now wit the knocking noise plus these 2 bad cylinder readings wat should be my next step? How can I tell If I damaged anything internally because it was knocking? This sucks and I'm pissed

[Rubyz32]

I would do a leak down test just to be sure. Teaspoon of oil in each cylinder and then repeat the compression test. Really sounds like a fuel pressure issue to me though. Have you checked the pressure yet?

[diplomat 1]

I will try the oil trick tonite but I'm still concerned bout the knock. I haven't checked fuel pressure yet but I did take the hose off after the fuel filter and seemed like fuel was goin thru just don't know how much.

[Rubyz32]

Any new findings? I'm curious if you figured out the knock?

[nissanfreak12]

Don't want to be that guy, but I would be concerned you may have blown a head gasket between cylinder 4 and 6. When you do a leak down test, and I would, leave the spark plugs off of 4 and 6 and listen to see if it leaks to the other cylinder. If it does, not good news. I hope its not.

One way you can check to see if you have rod knock is by unplgging the injectors one by one. If the knocking stops on that one cylinder, then thats the cylinder with the issue. Its a long shot but if you have a blown head gasket in between cylinders, its possible the one could have blow by that offsets the other cylinder causing a knocking sound.

Is the knocking sound rhythmic? or sporadic? If rhythmic, its a bearing. if Sporadic, possibly what I stated above or not really a knock so to speak. Let us know what you find.

[diplomat 1]

Sorry guys haven't got back to your. So this is where I'm at. After I did the first compression test I did another compression test. I realized I did the first one wrong cause when I did the test on cylinder 2 the spark plugs for 4 and 6 were still installed. Anyway I did it again and got 60psi out of cylinder 4 and 90psi out of cylinder 4 and 6. So I ripped into the motor after o found out my cas,tps and maf was working. Timing lined up perfect and I read that the intake sprocket locating pin might shear but neither was the case. So now I am stumped. I am pulling the head tonite. What should I look for for signs that caused the knocking and low compression? How can I check if I have a bad oil pump? How can I tell if the piston rings are causing bad compression? I will get the head off tonite but any more advice would be great thanks alot

[nissanfreak12]

The only way to check for rings is to measure. Oil pumps normally don't go out, just the way designed, do they yes, just not often.

Checking for knock, need to take the oil pan off and inspect the bearings.

Low compression could also be from your head like valves not seating properly. Before you take anything apart, do a leak down test, this will tell you where the low compression is coming from so you will have a better idea.

[diplomat 1]

We'll I'm way past not takin anything apart the motor is coming out today. Hopefully something stands out and can lead me to seeing wat my problem is

[nissanfreak12]

When you take off the head, you will know if its bad.

When you take off the bearings, the bad one will have grooves in it. Good surface will be very smooth, where your fingernail will not grab going across it.

[diplomat 1]

So I got the drivers side head off and found that the head gasket wasn't the problem I didn't see any breaks in it. I read on z32 wiki that the 1990 head have intake valve problems because the valves were made wit not inferior metals and I found that my head has those same "30" stamped valves. I'm hoping that that's the issue. I procrastinated a lot today butotor is ready to come out first thing tomorrow but here is a video of the cylinder walls and Pistons. Doesn't look like he valves hit the Pistons. Tomorrow I am going to take the pan off and see if the rod bearings are good which I hope they are but then I don't know where the knocking Came from. Here is the video http://youtu.be/hGbFYfyfGEw

I also took the camshaft off the driver side head and the first thing I smelt was burnt oil. Me and a friend both smelled it and thought maybe the oil pump was on the way out cause the cam shafts didn't look bad. I'm hoping tomorrow I can find out what's really the issue any advice would be helpful thanks

[diplomat 1]

A little update if anyone is reading this thread still.I finally got the motor on a stand flipped it upside and found the number 6 cylinder has a spun bearing. I took a video if anyone wants to watch it.
http://youtu.be/A4QTiknfKOM

As u can see the bearing is shot. My question is what are my options here? Cut crank and get oversized bearing? Is it possible to just replace the bearings and put the motor back together? I need advice cause it's either i spend money on fixing this one or buying another motor. Need help thanks

[nissanfreak12]

Sorry to hear about that, bum bearings are never fun.

Dont just install a new bearing withing grinding the crank, it will start knocking in a couple thousand miles, if your lucky.

If you have a bad bearing and compression on one side, either rebuild, or find a lower mileage engine, or one that is running. Another thing that could cause low compression in the one side is the timing belt skipped a tooth or two.

Personally, I would rebuild the engine. It will cost around 2k to properly do it. If you want to chance it, get a used engine, looking around 500. I have had really good luck on used engines, but these are cars I am selling and not keeping. If I am keeping the car, I will rebuild. This way you know it is good to go. If it does fail, only person to blame is you.

[diplomat 1]

Thanks bro but unfortunately I don't have the money nor time to do a full rebuild at this time. It sucks to say but I am headed for a used motor. I know this is a long shot but anyone in the Long Island ,ny area got a used 5 speed na motor for sale I am very interested. Funny thing is I had a motor that came wit the car that already had a spun bearing but the heads were just rebuild and brand new intake valves.guess I will just sell them. How much does a good motor go for? What's a good price range? Thanks nissanfreak

[nissanfreak12]

You can also use an automatic engine and one from an Infiniti J30. Just use all your sensors and stuff from your engine..

Go on classifieds, craigslist, ebay(just search within 50 miles of you), they are all over the place. You can use a TT engine, just use your manifolds not the TT and turbos. It wont be as fast because of the lower compression, but will get you going. Thats if you can find one cheaper than an NA engine.

[itsa300zx]

Too bad you are so for away. I have a NA engine from my swap on a stand, Shipping will be too expensive:(