Power Loss after 25 min.

#1

I have been fighting a problem with the Spec Car since I built it. I can run a practice session or a qualifying session and all is OK. These sessions typically only last 15 to 20 minutes. Now when I am in a race after about 25 minutes the car seems to lose power. It just seems to have now power for pulling out of the turns which causes exit speed to go way down as well as it does not seem to accelerate as fast down the straights. I have replaced the following trying to fix the problem:

AFM, TPS, CTS, O2 sensor, Injectors flow tested and rebuild, and fuel pump.

During the build in early ’07 I replaced the following:

Distributer, plugs, and wires.

The car is an ’88 but not sure what year the engine is. I am running a 173 ECU and normally run Mobil 10W30 but have also run Mobil 1 15W50 which I have now switched back to. I have a new coil on the way but don’t have any more ideas. It seems that it is a heat over time thing. The Dyno in July showed around 156 HP and 152 TQ so when cold it is not too bad. Simon and I had just about identical curves.

Anotehr problem which may be related is that it has a warm start problem. The car will fire fine when cold after sitting overnight but after sitting for about 15 to 20 minutes it is hard to start.

Anyone have any ideas?

Thanks,

Michael
#36
Great Lakes Region

#2

I was going to say coil. You have one on the way hopefully that will fix it. My coil erupted in a black ooze at RA last Aug. They really don’t like the heat.

#3

I had a flat spot at about 4800 - 5200 that only showed it’s ugly head after about 30 mins of hard driving. I took the car to a couple dyno guys. At the end of the day both the ECU and the AFM were weak. Buy a few used units, take them to a dyno and mix an match until you get the best result.

Don

#4

When the car is doing this, is there any sputtering or missing? If so, this might be a fuel starvation issue. If your’s is an early 88, it would have an in-tank transfer pump and an external high pressure pump. A weak transfer pump or a collection of sediment in the tank could cause problems as the level in the tank dropped.

#5

I don’t know much about E30s, but from what I understand some cars pull timing when the motor reaches a certain temperature (like 230). What’s your temp like? Does it get worse when you stay in the draft?

There’s a pretty lengthy recent thread on specmiata.com about the later cars (99+) flagging at the end of long runs.

Steve D.

#6

Not sure if this tale will help, but you never know where an idea will come from. Last month at CMP my car seem suddenly lose maybe 15% of it’s hp. And it sounded awful. This happened about 15min into an enduro. The nature of the sound didn’t point to an obvious cause. It didn’t sound like it was missing, it just sounded, I dunno, sick.

I brought it in and we gave it a quick look over in the pits and didn’t see anything obviously wrong. So I took it back out. 10min later I hit some rough gaters at a turn exit that rattled my fillings. And suddenly the hp was back.

I never figured out what the problem was. It’s had 4 track days since. The only thing that I can imagine that would behave like that (where a bump fixes it) is a loose connection of some kind.

#7

Mike,any misfire associated with the power loss? You don’t mention any so I’ll not go down that list of fixes.

Laura,s car had a high rpm “strangle” problem.Felt like fuel starvation, but it turned out to be an AFM. I’ve chased problems all of this year and mine turned out to be a loose crankshaft vibration dampner (goofy miss at high rpm caused by a sloppy signal).

Loofs like you’ve done all of the obvious. If it just falls off on power I’d look the direction of fuel. Matter of fact-change the fuel pressure regulator.Yeah that’s it as I just finished chasing that problem too.

Call and I’ll discuss the trial-and-error on the regulator.

Regards, Robert Patton 800-755-1715

#8

All I have is the stock temp gauge. it reads a tad below center while on track. It does feel like something is pulling the timing though.

Michael

#9

The car is a late '88 with only one fuel pump and it is in the tank. I have never felt any mis fire or sputtering. It is just noticably slower.

Also, it has a adjustable fuel pressure regulator. It came off my ITS car and seems to be working fine.

I am thinking about the evaporative purge valve. I was reading in the Bentley’s and it was talking that a faulty valve would cause hard warm starting. I have this. I wonder what else it may cause as time and heat goes by. I can not find this valve on Real OEM or on any of the parts websites. Anyone ever replace this valve? I am pretty sure it is the thing under the intake with a rubber tube and an electrical conector.

Thoughts?

Michael

#10

A bad purge valve that’s stuck open would be an intake leak and could cause starting problems. I don’t see it causing the loss of performance you describe. As a quick check, you could pull the valve and plug the nipple in the throttle body.

#11

After being way down on power this past weekend and even having Simon drive the car and say it is down on power, I did a compression test and leak down test on the engine. Dry compression test shows an average of 95 PSI with a high of 100 and a low of 90. At least they are all in the same range. The Leak down shows an average of 17.8% with a high 20% and low of 15%. Again all in the same range and not the worst numbers. Last we did a wet compression test where we put 3 squirts of oil in the cylinder. This showed an average of 130 PSI with a low of 120 and high of 140. Still in the same range.

I am confident that the valves are OK and that is must be the rings. My big question is, is it possible to replace the rings and have them seated before Nationals? I don’t want to replace the rings only to end up worse than I am waiting for them to seat. I may or may not do the bearings as I may just bite the bullet in the off season and rebuild with overbore. Anyone have any experience with just honing the cylinders and putting standard size rings back in?

Other options are to just stay home for nationals and save the money for the rebuild or to just go and accept the fact that the car is not going to perform at it’s best.

Ideas?

Thanks,

Michael
#36
Great Lakes Region

#12

No mention of the throttle cable issues?:laugh: