Hi,

Am helping out my friend with 94 FD. He had a broken apex seal. We are trying to figure out why.

It has stock JDM DP and a midpipe. Straight through muffler, and K&N Replacement filter. Before the seal broke, it was a perfect 10-8-10, with solid power. Our tests with a GTECH were giving 218RWP on average and the 1/4 mile was 13.8s.

Nothing seemed to be wrong but then it just broke.

I initially thought it could be to a bad tank of fuel and/or no premix when he was pushing it on some drags. Or even changing gear slightly in the red without realising. I thought that it was unlikely but could not see anything else which was more obvious.

THEN....

The engine was stripped down and 2 things were discovered...

1. HEAVy carbonisation on the rotors.
2. The o2 sensor was not connected (During his last service, a mechanic only connected 1 wire which was to his AF meter. It was not connected to the ecu.)



SO..

Are these 2 dicsoveries connected? and if so, could the carbon build up cause detonation, either due to a hot tip of it on the rotor, or the increased compresion ratio as the volume of the combustion chamber must have been reduced.

Lets hear your thoughts on this. If it's not the o2 sensor, he will have to look for other causes of the seal failure.


Thanks guys

 

Does he have an ecu? I know you said he had a perfect boost pattern but without some serious wastegate modding, I hardly believe that the boost wasn't getting above the 11 psi mark. That would be my guess anyways, he over boosted, detonated and lost a seal as a result. I think the O2 sensor could have led to the carbonizing but who know, I have never run my car with the sensor unplugged. The carbon could have aided in the detonation but I highly doubt the reduced chamber area had anything to do with it. Clean the rotors, replace the seal, throw it all back together, check your fuel pressure, slap the retard who forgot to reconnect the sensor and go on with your lives. Some mysteries are just that, mysteries.

 

the factory ecu uses the o2 sensor to adjust idle trims and economy, the only negative effect he could have had from that being unplugged was bad gas mileage.

 

The ecu only uses the O2 voltage at idle and part throttle. At WOT it is completely ignored in favor of the preprogrammed fuel maps. Unfortunatly Narrowband o2's like we and most cars use, are terrible inaccurate and slow to respond, too much so for accurate WOT readings. The ECU's maps are programmed to run rich at idle and cruise and trims the fuel down in response to the O2, so if you don't have it connected the car will run richer, and the abundance of unburned fuel will speed up the carbon deposits.. If the carbon is built up enough it can A: create hot spots causeing preignition, B: break off chunks and cause damage.

I have not had my o2 sensor hooked up for a while, but I run it to redline at least once every time I drive it to try and keep it clean, as well as occasionally cleaning out the combustion chambers with ATF.

I hope this helps.

25,000 miles and going strong.

 

Downpipe + midpipe + cat back + intake = blown motor eventually if you are running the stock computer

 

Yah you'd be overboosting like hell with that set up and prolly running lean.

 

some cars would be, some dont get the creep. mine was rich as hell on stock ecu with no boost creep. with those same mods.

 

I still blame the mods... also to be considered is what milage did the motor have on it?

Even for a spit-second it could of spiked on shifts upwards of 13-14psi but the boost gauge may not of even registered it, that would still be enough to cause potential damage...

~Kris

 

Played with fire and got burned. Another testament to the 2 (or 3) mod rule.

 

Here's a testament AGAINST the 2-3 mod rule, but I still wouldn't reccomend it. See sig below. 30,000 miles, stock computer, 7psi creeping up to 10psi when its cold out.