rb26dett compression test results, misfire

[Vkoslak]

I seem to have an intermittent miss at idle, I have spare coil packs and igniter from the rb20 I had before. Did a complete swap over, including spark plugs, no change.
So I decided to compression test the motor. This is the first compression test I've done on this motor since getting it running.
My gauge is in 5psi increments so these are approximate.

Results:
Cyl1: 150psi <- oh noes
Cyl2: 170psi
Cyl3: 170psi
Cyl4: 185psi <- wtf?
Cyl5: 175psi
Cyl6: 175psi

I did #1 several times to make sure. Always got 150.

The 185 on cyl4 confuses me though.
I did get this motor as a collection of parts in tupperware bins and not as a motorset. Metal head gasket was on it when I bought it.

I'm going to have to drive it like this for about a month until I can afford a new daily driver.

I think my next step is to have it leak down tested or smoke tested to see why cyl1 is low. My guess is valves. However I don't think this is low enough to cause my intermittent miss at idle.

At least I got it to pass the emissions test for inspection.

[supermario680]

Its just sightly out of spec, the max difference is like 10 or 15% so that would be around 17 psi compared to your 20 psi difference.

My bet is rings, you can put a tablespoon of oil into the cylinder and see if the numbers go up significantly. If they do then its bad rings.

I wouldnt bother with an actual leakdown tester with gauges and stuff, just pressurize the cylinder and just listen for where the air is escaping.

Intake = intake valves, will get louder when you open the TB's

Exhaust valves = exhaust obviously

Bubbles in coolant = HG

Out the dipstick = rings

[RRRRB]

compression seems good enough if the miss is interminttent its likely something else

[Vkoslak]

I've been trying different things. There is something going on with fuel pressure.
If I put it at the 35psi off vacuum, it leans way out. I also suspect there may be an issue with the coil pack harness. I could use the one off of my old rb20 setup, but I rewired that one for the j30 igniter. I saw one on ebay fairly cheap. I will try to get a smoke test going on this weekend. If the test is being made available to me, I see no reason not to use it.

[Vkoslak]

Here is a nissan datascan log from the other day's drive home.
http://www.vkoslak.net/~rjames/RB26DETT/20122151840.zip

[julio]

I've been trying different things. There is something going on with fuel pressure.
If I put it at the 35psi off vacuum, it leans way out. I also suspect there may be an issue with the coil pack harness. I could use the one off of my old rb20 setup, but I rewired that one for the j30 igniter. I saw one on ebay fairly cheap. I will try to get a smoke test going on this weekend. If the test is being made available to me, I see no reason not to use it.

It should be at 43.5 psi (3 bar) off vacuum, 34-35 psi with the vacuum connected assuming you are pulling normal vacuum.

[Vkoslak]

Yes, you're right. I said it wrong. I was setting it to 43 ish. It's a pain in the arse to put the gauge in line and then take it out again. I was thinking about trying to find a nice one I could just leave in line all the time.

[julio]

Yes, you're right. I said it wrong. I was setting it to 43 ish. It's a pain in the arse to put the gauge in line and then take it out again. I was thinking about trying to find a nice one I could just leave in line all the time.

Ok, otherwise it would have made sense that it leaned out when you set it to ~34psi at idle without the vac connected. It is definitely a good idea to get a gauge installed permanently, it makes troubleshooting fuel problems very easy. Especially considering you have an rb26 and will probably be modifying it/taking it to the dyno. Being able to troubleshoot on the dyno quickly can save a lot of time/money and frustration. Saving 1 hour on the dyno to troubleshoot a fuel issue (failing pump, poor pump voltage, clogged fuel filter, etc...) would pay for the gauge for example.

As the other member suggested, just poor a small amount of oil in the #1 cylinder and see if the compression raises to that of your other cylinders. Its probably the easiest check and most likely culprit.

[Vkoslak]

Any recommendations on the fuel pressure gauge? Under-hood or in-cabin?
If in-cabin, I was thinking of going with the defi unit as I already have their coolant temp gauge. It's pricey, but compared to what I've spent so far...

If under-hood, I was thinking just getting a inline adapter with a 1/8 npt and whatever the local O'Riellys carries.

[julio]

B&M and Aeromotive make nice inline options.

[Vkoslak]

I've been poking at the motor some, and I noticed something odd. If I unplug the negative battery cable, even for a few minutes, when I hook it back up and drive the car, it drives great. Hitting 8.5 lbs of boost. After a few miles, it suddenly falls on its face and it's hard to get the car above 50mph and that's only with very light throttle. The more throttle I give it, the more it doesn't want to go. If I stop and unhook the battery, I can drive it again. I had my laptop hooked up last time I drove it, and even if I do a "clear self learn" it still runs like crap. It's only once the battery has been disconnected that it drives better.

I'm leaving it in the garage until I can figure it out. Three years into rb26 swap after I ruined my rb20 at a hpde and I'm still not able to drive it. When I did the rb20 it was up and running in 6 months.

I've checked the tps, set to .44v on the consult readout. I even tried a different tps. I somehow ended up with two of almost everything. I picked up another ecu, but it is a r33 ecu and wont run on my harness. The ECU I'm using is chipped, but I put a stock rb26 bin on it. I see no plugs for stock wastegate solenoid or boost pressure on the harness (RawBrokerage), so I'm assuming at this point that those things don't matter for how the ecu runs the engine. I'm using a 300zx coolant temp sensor for the coolant and the air temp. Local expert on Z's suggested the ohms range would be different on an air temp sensor vs a coolant sensor, so we put a maxima air temp sensor on the connector to see if that made a difference. It seems to idle better, but it acts the same when you are driving as when using the coolant sensor.

My old wideband harness got too close to the turbo on the old motor and became very crunchy, so I am going to buy a new one to try and give me another tool to figure this out. I was hoping by using all stock pieces I would avoid some of these headaches.

[Vkoslak]

I did some googling, ran across this: http://forums.gtrcanada.com/gt-r-tech/5 ... moval.html

So I ordered a oem boost sensor for the rb26. It won't be here for a couple of weeks, but I guess that gives me time to get the wires for it run to the ecu since I don't see a plug on my converted harness. Good thing I have an extra harness that I don't mind ripping apart. I should do the wires for the air-con while I'm at it.

Also, when I look in nistune, there is a duty map in the rb26 ecu bin for high boost. I assume if the boost sensor is not hooked up, that duty map is not used.

[Vkoslak]

I got my sensor delivered, but checking the wiring diagrams, it doesn't connect to the ecu. Sunny beaches. Anyhow, I also got in my aeromotive fuel pressure gauge and an inline adapter from concept z. Once I get that in and everything set correctly I am going to start double checking sensors using the service manual data. I suppose there is a chance it is the ecu itself that has the issue, but I want to check the sensors first. So close, yet so far.

[julio]

I don't know about everyone else, but my car works find without the boost solenoid connected.

[Vkoslak]

I put in the Aeromotive fuel pressure gauge and the reading was different from what my old unit was telling me. It was about 10 psi too low. I corrected it and took it for a nice long drive. No more lack of acceleration issue. I did notice that the on vacuum pressure was only about 2 psi different than the off vacuum. That might cause it to run rich at idle. I have a T in the vacuum line from the stock location. One side goes to FPR, the other side goes to a 300zx fuel dampener I put on the system since I didn't have an rb26 dampener. FPR is a nismo unit I got from courtesyparts.com. I think it is a sr20 unit but I had used it for years on my rb20det with no issues. At least I can finally drive it. I did finally get a daily driver so it won't pain me so much when I need something done to my rb car. Daily is a near stock 89 240sx se hatch with 200k on it. At least it has AC.

[Vkoslak]

And now it runs like crap again. I didn't change anything. The only thing that has changed is the weather. It was mid 70s the other day, and now its mid 40s.

[Vkoslak]

Update!

I went to a local shop that specializes in Z's (Z Clinic) and worked with Kevin to try and diagnose the issue with the loss of power. I should note the loss of power is accompanied by an oscillating honking noise and recirc valves opening anywhere from 2500-3500 rpm. The pattern does not get quicker or slow down with rpm. We started with eliminating the recirc valves as a source of the problem. Pulled them out and capped the holes left before and after the intercooler plus the lines to the recirc valves themselves. Took it for a test drive, no power loss, but the honking noise remains and is now louder. Next thought was that the wastegate actuators are not opening together or that one or both have an issue. I had a blitz boost controller I had not installed yet, so we put that in keeping the boost low since I'm on oem turbos. While that certainly helped with acceleration, and boost was kept right at 10psi, the honking noise remains. It was getting late so I packed up my stuff and went home. At least I can maintain highway speeds.

Since then, I had a loud backfire, and now it sounds about like an exhaust leak from the rear three cylinders. I can still make boost, I still have the honking noise, but the rear three cylinders are really loud. I don't see any obvious leaks. I'll have to take some bits off to make sure, but my suspicion is the rear oem turbo has went. I had a similar experience with the rb20. If it has let go, I'm seriously considering single turbo conversion. Two turbos is two times the mess.

[julio]

If you go single, I have a set of 870cc injectors, 4" lightning maf, and tune I could set you up with.

[Vkoslak]

Cool, I'll keep that in mind.

I'm hoping it's just an exhaust leak. I was planning on going with n1 turbos eventually since I'm more into response than big horsepower numbers, but I've not been fond of dealing with two turbos around my steering column and weird honking noises.

[julio]

Cool, I'll keep that in mind.

I'm hoping it's just an exhaust leak. I was planning on going with n1 turbos eventually since I'm more into response than big horsepower numbers, but I've not been fond of dealing with two turbos around my steering column and weird honking noises.

I think medium sized singles (60mm-62mm inducers) are just as responsive as twins. Something along the lines of a hta3586, 35r, 6262, etc... can get you 1 bar by 3500-4000 rpms. Especially so if you get a spool valve or twin scroll setup. Most people would kill for a 4krpm power band like that. Its the 67mm turbos where boost threshold and response become a real issue.

[Vkoslak]

I finally got around to poking at the motor some more. Definitely an exhaust leak. Gasket for dump pipe to down pipe from the front turbo gave out. Did the compression test again. This time I added oil to cyl #1 and it raised it right up to match the others. So that means rings. I also put on aftermarket wastegate actuators from rawbrokerage. I still have the honking noise, but it hits 10.5 psi with the ebc set to 0. Seems to come on boost quicker, so that's nice. It did pop the dipstick yesterday for the first time. Now I need to decide what to do. Rebuild the 26? Leave it as is until I figure out the honk? Sell all my 26 stuff and go back to rb20? I don't really see myself selling the 26, rb stuff seems impossible to get, and impossible to get rid of once you have it.