Power drops after several hot laps???

#1

Group:

At VIR, my car seemed to drop-off in power after about 5 hot laps. This is based on several factors during the races, and the fact that a few guys whom I respect highly due to their outstanding racing, mentioned that my car seemed to lose power after about four or five hot laps during the races. These were unsolicited comments that were made to me during the weekend.

I don’t want to be the guy who blames his car for a poor finish but after staying with the front group for a few laps, I couldn’t quite keep up going down the straights.

If it is the car, does anyone have any suggestions? The engine ran at about 195 degrees (in the middle on the stock gauge - I also have an Autometer gauge) in the cooler morning session and at about 205 (just to the right of middle - stock gauge) during the races. While following the lead pack for a few laps, it crept up to about 215, or about 3/4 on the stock gauge. I have all water and Water Wetter, and a gutted thermostat.

After I started dropping back from the lead pack, the temp went back down to about 205 to 210. I’m thinking that it is still running lean, as it did on a dyno at the end of last season. I need to get it back on the dyno again; the dyno guy said his small-block Chevy drag motors would melt at those a/f ratios.

When adjustable fuel pressure regulators were legal, I installed the Ireland unit but it didn’t affect the a/f ratio, according to the dyno a/f ratio probe. swapping ECUs on the dyno didn’t improve the a/f ratio.

I installed a new air flow meter earlier this season.

I’ve been told to replace the coil because it can get hot and not work properly. I have also been told that a bad temp sender that feeds the ECU, might make it run lean. The O2 sensor is fairly new and is plugged-in.

On the dyno, the engine does make more power when the a/f ratio gets more rich so I need to find a legal way to get the engine the fuel it wants. I haven’t checked the fuel pump/fuel pressure but it seems to be working ok. The car won’t starve for fuel until the tank is almost empty.

My plan includes a new coil, a new water temp sender for the ecu, possibly a wide band O2 sensor, and a leakdown test.

Any thoughts from the engine experts?

TIA!

Carter

#2

Put the thermostat back in.I need to look at a coolant flow diagram, but my belief is that it not only opens when hot (its inherent design) but also directs coolant away from the cylinder head.

Others correct me if this is not right.

I would suspect a lean-out or problems with the AFM.We’ve had problems with those before. I can also check my “stuff happens” file.

Regards, Robert Patton

#3

At WOT, the DME is using built in maps and the O2 sensor data is ignored. Only engine temp, mass air flow, and rpm are used. At least that is what I’ve been told.

Since a swap of AFM’s didn’t cure the lean condition, that only leaves the engine temp sensor, DME, and fuel system as possibles. In the past you’ve swapped the DME to no affect, so now we are down to the temp sensor and fuel system.

You could test the temp sensor, or just replace it. I’d also recommend running the fuel pressure and delivery tests.

You could have a problem with the injectors. They could be dirty and limiting flow or there could be an electrical problem with the injector power or grounding path back to the DME.

#4

Carter, I checked my notes from March to May.

Advantage of having two cars: you can swap known good parts.

Disadvantage: twice as many problems.

We had the same lay-down problem with the 911. Swapped AFM to the 241. Then the 241 had the problem. Swapped with an e30 street car and the problem was solved.

Regards, Robert (I can’t fix 'em, but I can swap parts) Patton

#5

Definitely reinstall the thermostat. Where the arrow is indicating “up” drill a 3/16" bypass hole…that allows the temp gauge to read temp even if the thermostat is stuck (saving you a cooked motor).

I really don’t like to run my motor over 185 degrees, so I have gone to a Ron David double pass radiator. Inlet and outlet on the passenger side, use “i” bottom hose, and “e” top hose and everything bolts up. If your radiator is not under a couple of years old, replace.

I agree with others that you are running too hot. Without a thermostat, water from the back of the head circulates to the intake side of the water pump (I think) and is not cooled before going back into the block/head. You can plug the lines at the spigot both on the back of the head and the thermostat housing…temps will drop drastically just doing that. Before changing anything, fix your plumbing.

The leaner the motor runs, the hotter it will run…sort of self perpetuating. I don’t understand the rules not allowing an adjustable FPR. To me that is a no brainer. It is much cheaper than trying 10 different ECUs to find one good enough. At some point, people are going to start burning down motors…a 135$ adjustable FPR is certainly cheaper that a motor. Chuck

#6

Carter,

I have similar problems with my car. While it still runs lean and it is till slow down the straights vs. the fastest car in the region, it is tolerale. I spent a horrendous amount of time at the dyno trying to figure things out and learned a couple of things.

Be sure your throttle position switch is working. This is what tells the DME to ignore the 02 sensor and feed full rich fuel (at least as I understand it). Mine was broken and even at WOT the DME was adjusting A/F ratio via the 02 sensor.

Try differnt injectors. Plus don’t assume that reworked injectors are perfect. I had some rebuilt rejectors form RC Engineering that did not work right out of the box. After 20 dyno pulls, all new fuel delivery parts including fuel pump, and three different AFMs (one known good from John Black’s high HP hotrod) the car still only made 130HP. I put the 19 year old stock injectors back in and it runs fine. I was out of time and money by this point so I did not go back to the dyno. The car ran pretty good at the last event but it still does not have front runner power.

So start with TPS and think about the injectors. I am goint to try the other tricks people have offerd here too.

Don

#7

For fuel injector cleaning I use this guy, cheap and good service.

http://www.cruzinperformance.com/fuelinj.html

#8

Ditto on cruzinperformance - have used them twice with good results. Same on thermostat. TPS is easy to replace, but check manual because you have to index to a specified resistance value.
Ed

#9

The TPS is correctly installed if the idle switch closes when there is .020-.060" of clearance between the idle stop screw and the throttle.