Compression and Fuel Issues

[Guest DoTheDrew888]

Ok heres the story. I just rebuilt my L28 and swapped over to flattops with an N47 head. I finally got it running today but it wouldn't idle. I went through the regular checks to see if I could figure out what the problem was. Got around to pulling out the spark plugs and they all looked good except for cylinder #3. In cylinder #3 the spark plug looked brand new so I know im either not getting fuel or not getting spark. I checked the injectors and they seemed to be working fine but im not positive yet. Well, I moved on to see if anything else was wrong with her. I checked the compression and had 175 in all cylinders except #3. Cylinder #3 has a compression reading of 200! How the hell did this happen? I pulled the valve cover to make sure the valves were opening properly and they seem to be fine. What else could cause the compression to be so high on just one cylinder? Could the fact that there was no combustion in cylinder #3 have anything to do with the high compression?

 

Thanks for any help you guys can give me. I have pulled of the fuel rail and will be cleaning and pressure testing all the injectors tomorrow just for good measure. I already tested the fuel pressure and it is fine so that isnt the problem. Im pretty sure Im getting spark in cylinder #3 but I am not positive. I will report back tomorrow with more info and hopefully some good news.

[Z Driver]

To verify spark, unplug the spark plug wire from #3 and hold it close to something metal to see. If a spark arcs from the wire to the metal, you have spark.

[Guest DoTheDrew888]

Yeah I know, I assumed it was the fuel injector though because all other cylinders were getting spark. It is brand new wires, rotor, caps, and plugs so I really doubt it is the spark. Im almost positive it is the injector.

[technicalninja]

My guess is the valves in cyl 3 are out of adjustment slightly (not slightly broken in) and you are not getting any fuel in cyl 3. If your car is F.I. I would guess that the injector harness was not making good contact and the injector was not pulsing. You can verify that the injector is pulsing with a medium screwdriver. lay metal end on injector and put your ear on the plastic handle- If it is working you will hear click every time it fires. Real common to have to work/clean/bend inner metal contacts to make a good connection.

Valves being just a little loose can increase compression drastically. Once I checked a 74 260Z's (original never worked on engine) compression before and after a valve adjustment. I am very experienced at setting valves and did a "blueprint" style adjustment. Exactly in center of range. Compression varied 20 lbs across the engine before adjustment. After adjustment compression was 174 in all cylinders-exactly. I was so shocked that I redid the test twice with my Snap-on gauges and borrowed another tech's gauges and did it again. Although his showed another reading (I've never found two comp testers that read exactly the same) all cylinders were exactly the same on the one gauge. I have since tested comp right After V/A on other heads and other brands (not limited to Nissan) and can say that valve adjustment has more effect on compression readings that any other factor (as long as the engine is basically sound).

One chamber has not seen any fire in your engine- no real break-in has occured. If it's compression was not different then the rest I would be suprised.

Not to worry. I would not take injectors out until having properly diagnosed cause of #3 no fire. Only after checking for spark and injector "click" would I remove injectors.

The last Z I resurrected did require 6 fresh injectors as even though the all were clicking. Their patterns (fuel cone at injector tips) were all streaming (not forming a cone, forming a stream as if out of a water gun)

I would not do another compression test until I had taken the engine through a few complete warmup cycles, put a few miles on it, and re-adjusted the valves very exactly. Most valve bed in will occur in the first 15 min of operation. I almost never have to re-adjust one of my engines after the second adjustment.

I would not operate the engine for very long with # 3 misfiring.

Hope this helps

Rick

[olie05]

if your engine is not firing on #3 and the cylinder is just moving up and down, oil will just sit on the piston rings instead of getting combusted with the spark and fuel. That oil could cause a higher reading than normal.

 

Do you really feel a lack of power as if your engine were running on 5 cylinders?

 

maybe you just have a really clean cylinder...?

j/k

-Oliver

[Nismo280zEd]

You should really verify spark first... trust me.. I've had the same problem with my 280.. and I had a wire that was bad and wasn't wiring (and they were barnd new NGK wires.. 1 month old) just plug un plug the wire.. hook it up to a spare plug and lay it on the valve cover, you'll find out real easy if you have spark of not (and you can see from inside the car while your turning it over) always a plus! Since it's EFI I would check timing and vacumn as well. I know on the 280's the pressure reg on the manifold is controlled with vacumn.

 

Also this might sound retarded but.. have ya checked your spark gap?

-Ed

[Guest DoTheDrew888]

Yeah I checked the spark gap haha. I also checked for spark and it was ok. So I pulled off the fuel rail and pulled out all the injectors. Guess what?... #3 is clogged. From what I can tell it has been clogged for some time now, which would explain my very low baseline dyno results. Anyway, Im looking for a new injector and will put it all back together soon. I am going to readjust the valves and hopefully that will solve the compression problem. But I found another problem yesterday, apparently I didn't do a very good job putting in the freeze plugs cause they are leaking. Its always something... But, that is the least of my worries right now.

[Guest DoTheDrew888]

Update:

 

The car still wont idle and apparently my radiator is shot cause it keeps overheating. I know the water pump and thermo are working correctly so it must be the radiator. Anyway, i've tried everything and the freaking car still wont hold a steady idle. It will either stall or surge to around 2000rpm and stay there. I can't figure this one out. Im thinking there is a air leak somewhere into the intake manifold but i can't find one anywhere. Could this possibly be caused by vacuum line issues? I know it isn't the AFM because i've tried several different ones and the same problem occurs. Any suggestions would be very helpful. Thanks,

 

-Andrew

[COZY Z COLE]

Have you sprayed water into the engine comp.?? Water around the TPS will short it out causing the same problem that you have....

 

LARRY

[Guest DoTheDrew888]

Nope, I've cleaned the TPS 4 times now just to make sure. I also triple checked all the other EFI connections. I am getting spark and all injectors are working. Fuel pressure is fine and the AFM is working. I may have the vacuum lines connected in the wrong order, will that make a difference?

[Guest DoTheDrew888]

Come on guys, I need some help here. I've checked almost everything and I am not getting any results. Here are the symptoms so give me some ideas, i am desperate!

 

With idle set at regular position: The engine will hold idle for a second and then stall.

 

With idle turned up: The idle will surge up to 2000+rpm and then drop down to about 500rpm almost to the point of stalling. I believe 2000 is what the idle is actually set at so the drop is what is wrong.

 

So... the problem is the car isn't controlling the air and fuel ratio properly or it isn't providing enough of one or the other. I know that it is not the AFM or the TPS. Any idea on what this could be? This is very frustrating and I would really be greatful for some help.

 

Thanks a lot,

 

-Andrew

[JMortensen]

I'm thinking maybe a vacuum leak. Try spraying starting fluid around the intake hoses while the car is running. Flex the hoses around too. Sometimes a hose is cracked just a bit and when it flexes it leaks.

 

My other thought is a dirty throttle body. If that happens the engine will surge then stall. Just fixed a dirty TB on my father-in-laws 5.0. Paper towels and carb cleaner made a huge difference in the way that car runs.

[Guest DoTheDrew888]

Well, I know its not the throttle body because I cleaned that really well when I have the manifold off the car. So Im thinking it has to be vacuum or computer related. I've already tried the starter fluid idea and no luck with that but I will try again this afternoon. I guess I will play around with the vacuum lines and maybe try cleaning the ECU also. Would having the vacuum line hooked up in the wrong order cause any problems. I always thought that all the lines get equal vacuum so it didn't matter, I may be wrong though.

[technicalninja]

Improper vac line routing can have a serious effect on idle.

What year/ fuel inject/ mods?

Is #3 firing now?

Does it run OK off idle? Have you driven it yet?

 

If you applied man vac to the EGR it would destroy idle much as you described.

Vac line routing on most Z intakes is very simple and easy to understand but I need to know what year.

You can eliminate nearly all of the vacuum lines for testing purposes and the engine should idle fine with out them.

Needed lines:

FPR to manifold.

TB to dist (ported vac)

 

pretty much everthing else can be capped off at the manifold end of the lines.

Look closely at the rubber boot between the TB and AFM. They can get cracked down inside the pletes and you cannot see this unless you bend and streach out the boot.

[Guest DoTheDrew888]

It is an '83. But I figured out the problem. The line between the TB and the Charcol canister was cracked. Also, the valves needed to be worked in. After driving it for about 50 miles it started running fine. Thanks for your help guys.