I am working an my bosses 87 turbo II he just got, and he has a nasty missfire as well as some pretty hardcore bucking under boost. In first gear its ok then it bucks/hesitates as the boost comes up then clears up and feels like it hits a wall at 5500rpms, with a hard misfire that keeps the motor from continuing to rev till you shift the car then the whole cycle repeats itself minuse the intial hesitation.

I have replaced the spark plugs with no possitive results, the car had been sitting for 3 months prior to him buying it, it smokes pretty good on start up but most of the smoke goes away while driving. We were told the motor was rebuilt with 30k on the new motor, it has no cat, aftermarket plug wires, an HKS Fcon for fuel, and a downpipe and exhaust. The car does however make an strange almost misfire sounding noise as the rpms fall off when you let off the throttle and coast in gear, I know you guys here are very knowledgeable and can point me in the right direction.

 

sounds like there could be a number of things wrong, but when you say bucking while under boost I immediately think of the restrictor pill that should be in the vaccuum line going to the pressure sensor located on the front of the passengers side strut tower.
-John

 

That and/or the TPS needs adjustment. How much boost is the car running? Is there an FCD? You might be hitting fuel cut.

 

There is a clear plastic restrictor pill that is in place, also, if you don't go past maybe 50% throttle, and keep the boost non existant, it will pull fine all the way to the redline.

 

It's leaned out too far. Check the fuel injectors, fuel filters (there's on on the pump as well) and fuel ines.

I'd remove the Fcon and see if that helps as well. The misfiring sound on downrev is nearly normal. You need to remember that these cars have no pistons, so it's completely different ball game.

 

The car is running 10psi, correct me if I am wrong but the fcon also doubles as an fcd, if not where might I find the FCD wired into this particular car.

 

The FCD would be wired to the ECU which is behind a kickpanel on the passenger side floorboard.

 

I will check some of this stuff when I get off work, I am gonna do a compression test as well.

Oh and to the people who offered some advice I would like to say thank you, I have been doing alot of reading on rotary engines but I feel its best to talk to those with expierience, if this were a turbo honda or a dsm I am sure it would be running right by now as I have more than a little bit of expierience with those

 

Does the car have any mods? If not you shouldn't be getting 10 lbs of boost. It sounds like you are running into fuel cut. It's a safety thing that cuts fuel to the rear rotor if boost gets too high. Check and make sure that all the vac lines are on the turbo/waste-gate actuator, etc area and that the wastegate is working properly.

 

Yes the car has several mods, hks pfc-fcon, a downpipe, gutted cat, converted to single 3inch exhaust, alternator pulley, oil cooler, boost controller, intake.

 

Ok. It still sounds like fuel cut though. And another location for the FCD is plugged in at the boost sensor that's mounted on the front of the passenger side suspension tower. The one I have is just a small black box without any markings on it.

 

I think I found it it has a screw adjustment right there? or is that not it?

 

Unplug the electrical connector to the boost sensor and go for a test drive.
Carefully push the engine into boost.
Does it still buck like crazy?
If it doesn't, you were hitting fuel cut, and the FCD isn't working correctly.
Remember to plug the boost sensor back in after you're done testing!



-Ted

 

UPDATE *****

We used sea foam on the engine and replaced the 3 month old tank of gas, the bucking is gone and the motor feels like it has much more power, however we are definately still hitting a wall at 5500 rpms which is where it hits about 10 psi. We turned the boost down to 7-8 psi and it still does the same thing.

Now for the weird part, we found an exhaust silencer in a box of parts and installed it in the muffler to keep the noise down during our late night tunning session, with the silencer installed in the exhaust the car runs fine and pulls all the way to redline. We then removed the silencer again and played with the fcd and still kept hitting the wall.

Now a question for you guys, this car has a considerable sized leak between the turbo and the engine, how much backpressure is required for a rotary engine to operate properly, is it possible that when we install the silencer in the muffler its increasing the backpressure enough that the motor smooths out? I am familiar with turbo cars and understand under normal circumstances the more free the better after the turbo because all the backpressure the engine needs is found between the engine and the turbo in the exhaust manifold, however with this leak I think we may be loosing enough backpressure in the manifold that the car runs ok when we are suplementing the backpressure post turbo at the muffler.

Thanks again for all the help.

 

Its not the backpressure my man...

Fuel Cut fells like you are driving a washing maching on spin cycle with a lead ball on one side... its more of a vibration that a buck...

stock fuel cut is around 8.5 PSI.... if the FCD isn't working, it still could be hitting boost cut if you are referencing an aftermarket boost gauge (my haltech shows 10 PSI when the autometer gauge shows 8!!)

the reason it runs smoother with the baffle in the tail pipe is probably because its not seeing as much boost because of the restriction and exhaust leak combination.

I tend to agree that you are running lean.... but some 94 octane in that thing...

you REALLY need to get a wide band on that thing before you puke it

 

UMMMMM Sea foam gas additive or oil additive??????????????????????????

 

Seafoam as a gas additive, the car has a full tank of 94 in it, and the plugs are a nice golden brown.

We also pulled a can of sea foam into the engine while it was running through a vacuum line in the intake manifold and that seem to really help the motor run smoother.

Also this car has had all the emissions crap removed as well, it has a walbro 255 fuel pump and a bigger fuel line installed, so I can't imagine that is running lean although I definately won't rule anything out.

 

Guess you don't want to listen...


-Ted

 

You need to get the Rtech computer chip I also had fuel cut but it was at 6500 rpm. The upgraded computer fixed that and it runs a lot better now. I can't post a link cuz I'm dumb

 

Ok, well we unplugged the boost sensor and that make the miss much softer, we have changed the intake manifold gasket found a couple fuel leaks at the returns for both fuel rails, checked all the grounds and such. The miss occurs at 5psi no matter what. How often do the stock ecu's on these cars fail? Is that a possiblity.

Anyone have an idea where we should go next?

 

could this be a 5/6 port problem? I guess this car has a jspec motor in it from what we have found out.

 

turbos dont have 5/6th ports.

 

I had the same problem with my car at 6500rpm, just hits a wall. I removed the F-Con and FCD and it hit 10 psi all the way to redline with no problem. Then the engine popped

 

I had boght a used ApexI AFC II, When installed it would buck around 5500 rpm. Removed it and it was fine. I finaly boght a new one off ebay and it worked fine withe the same apexi harness, THe used apexi harness had some dead wires, that I fixed, so yhea...

That thing about the silencer you mention, has to lead to something. why would more back pressure remove the bucking? maybe it was running lean around that rpm, so it trigered the fuel cut, or it was running rich...


Gil