Thread: altitude misfire

92 stock SER used for rallycross. Tested it up here Friday prior to trailering down for an event and it ran okay. The engine is currently all stock. I towed it down and topped off the tank along the way with 100 unleaded to start bringing the tank octane average up because I've ordered a high-octane tune ecu which I should have soon. When I got to the race site as soon as I fired it up on the trailer it acted like it had one or two spark plug cables off. (Plug cables were tight.) After warm up it still misfired and after one practice lap I took it home. When I went to pull it off the trailer up here, the engine runs fine again.

Here, I'm at 2700 ft, the race site is close to sea level. The only thing I can think of is the mixture ratio is slipping and the altitude change makes it run or not run. So I assume there's a fault, but with the engine running okay up here, I don't know how to identify and troubleshoot it. Anyone have any ideas?

I would suspect the 100 octane fuel. Either it's purity or whether or not the motor can handle it. I've noticed some random problems running 94 octane in an otherwise stock SR20DE (stumbling and random misfire)

Higher octane fuel will burn more slowly than lower octane, all else being equal. That is why for higher compression motors, with corresponding higher combustion chamber temps, require a higher octane fuel so that the fuel doesn't ignite (ping/knock) prematurely. Too high an octane in a stock motor and the fuel will still be burning as it hits the exhaust manifold.

When this happens (I'm just thinking this through logically not from any first hand experience) you will have a high pressure in the exhaust manifold that will prevent the next cylinder from exhausting properly, and some of this still burning (and expanding) fuel will get trapped in a cylinder. The next cylinder will likewise not be able to exhaust properly, more trapped burning fuel, and the end result is going to be an extremely poorly running motor. Or worse.


What I'm puzzled by, though, is that this ran better at a higher altitude with the high octane fuel. Thinner air, less oxygen, should have made this problem worse, and not better. So I'm very confused by the result.

I have no idea why you are wanting to have a 'high-octane tune ecu' on a stock motor. That doesn't seem to make any sense, if you haven't modified the motor (increased compression, etc) to require the higher octane.

You're right, I have no need for higher octane in this stock motor. But as I said, I'm about to undertake mods including a 100 octane tuned ECU (advanced timing) and was preparing for that. All "authorities" are pretty unanimous on higher octane than required doing no good, but also causing no problems. Hey, maybe they're wrong. On the other hand, the tank still has 50% 100 octane in it currently and the motor is running fine. So too high an octane only hurts at sea level?

The fuel is a possibility, but I think more likely a hardware problem somewhere. The car has always been hard to get through smog testing and failed last time I tried. I should have paid more attention to why, but a trailered race car -- I really had no reason to keep it smogged, so let it slide. I should see if I can find that old paperwork because it might have clues.

If you guys don't come up with some likely culprits, I'll probably trailer it down to a tuner I know down near where I race. Hopefully it will act up for him and he can chase it down.

I've been thinking about this. You have slower burning fuel (high octane), and you have no problems at a higher altitude (less air pressure, less oxygen), and also the car was having problems passing smog test (new data). What I was thinking last night was that you may be having an incomplete burn of the fuel, and so what I said in my post above about the fuel still burning and expanding as it hits the exhaust manifold isn't happening at the higher altitude. The fuel has stopped burning (or mostly) by that point, so there isn't a problem with how the car runs. But drop it down to sea level, more oxygen/air pressure, and the slower burning fuel continues to burn as it leaves the cylinder, causing issues.

If you had problems passing smog, you probably already had issues with unburnt fuel that you are going to want to resolve. This isn't tuning related, it is mechanical. Dirty internals, clogged hoses, passages, or even broken parts. You will want to get these issues addressed before your car can effectively be tuned.




I went back through your other posts looking for clues, found this other thread which I had forgotten about. Could this leaking gas have actually been from the exhaust manifold or header area? You would only see this happen when those parts are cold, like when you pulled the car out of storage, or even just sitting overnight.

Thanks for your efforts on this. Your last point first since I have a short attention span. The leak turned out to be a combination of things. Some fuel was leaking from the rubber filter hose, causing the smell. The bigger pool was tranny fluid from a suddenly dead axle seal. And the power steering was weeping a little here and there. The car didn't like being parked two years.

Sorry for bringing in data in drips and drabs, but I did find some of the smog paperwork. When I first bought the car a decade ago, the seller was unable to smog it because it tested 50% over max for NO @ 15mph. But in my county, we don't test for NO, so it was not a problem and never corrected. After all, it ran fine. It is however a sign of some sort of problem. And given excessive NO, likely a lean problem. My local techs want me to run a fuel pressure test to check pump and regulator.

When I ran into trouble smog testing up here, the exhaust system was a broken up mess from the rallycrossing. Don't know why that killed the smog test, but after exhaust repairs it passed. Now it fails again, but the exhaust is torn up again. I have parts on order. I don't know if any of this is related to the current misfire, but I kinda doubt it since the misfire is new and all the smog and exhaust leak stuff has been around forever.

I could change out the fuel up here, but wouldn't have a way to check for improvement up here in the high country. Still, if I don't have any other smoking guns, I'll probably change out the fuel before I take it to my sea-level tuner and find out if the fuel change is the ticket when I pull it off the trailer down there. I'll know the moment it fires. -- it's that obvious. I'll bring a can of Torco with me so he can up the octane as required when he installs the new ecu.

EDIT -- by the way, I haven't stuck my head under the tank yet...is there a drain plug or if not, what's the best way to dump the gas?

This. Said much better than I could have said it. +1

Honestly it's probably not worth it. IIRC either you can pump/siphon it out, or drop the tank and drain it. Not easy work especially if it's half full.

Screw that idea, much more efficient to just disconnect the fuel line at the rail, and jumpstart the fuel pump relay. Takes 3mins tops to set up, and will drain the tank MUCH faster than a siphon would.

Yeah, I tried a siphon thru the fill and couldn't get the hose to the right spot. I thought I might pick up the fuel at the inlet to the filter and either pump it out or extend the hose and let it drain low. No hurry -- don't care if it takes time.