Need some help here, I installed a Kerry Hunter header w/Supertrapp a couple of weeks ago and it has leaned things out a bit too much.

I bumped up the fuel pressure to 34lbs which helped, but not enough. Bought a 0-1k potentiometer, put it in line with the CHT sensor, and the car is now hard to start. Does not matter whether the resistance is set to zero, 1k, or anything between. Once I get it running it seems to run ok, maybe too rich but haven't had a plug out yet to check.

Anyone know what the range the resistance should be in? Any other ways to richen up the mixture? Don't really want to diddle the MPS!

I plan to add a air/fuel ratio gauge this weekend, but if I can't get this right I may have to put the stock exhaust back.

Thanks,

....Doug

'74 2.0

Adding resistance to the CHT is just for warm up so thats your problem there. I'm gonna bet you will have to adjust the MPS to the exhaust. Not too hard to do if you can find access to a Wavetek and a dyno.
Geoff

I had heard that before, that the CHT was for warm-up only, but could not find anything like that on the Anders site.

What if it never thought is was warmed up?

I looked around the garage, no wavetek, no dyno.

Anyone BTDT? Who's running a header with a D-Jet?

Graci,

....Doug

Hi Doug! Welcome to the board. I'm no D-Jet expert, but I hope you get it worked out so you can continue to run the header.

Brad addressed this on Pelican a week ago.

"ballast resistance to the head temperature sensor has complex effects. For most ECU's, when the CHT resistance drops below about 100 ohms (corresponding to a head temperature of about 210 deg. F), the ECU interprets that the engine is fully warmed up and does not lean out the mixture any more. If small amounts of ballast resistance (less than 100 ohms) are added, the effect is that the warm-up mixture is richened, but the mixture when fully warmed up is unchanged.

If large amounts of ballast resistance are added (over 100 ohms), the overall mixture is richened, but with an unwanted side effect that the ECU continues to change the mixture even after the engine is fully warmed up. Since head temperatures vary considerably under different running and environmental conditions, this varying mixture can lead to some odd drivability problems (e.g. crappy idle right after hard runs, gets better as the motor cools, etc...).

This behavior is shown by the diagram below:
The CTC Vout is the control signal in the ECU that varies the mixture by head temp. Higher values of Vout correspond to richer mixtures. Note that the 037 2.0L setup is quite different, due to the different CTC resistance profile and 270 ohm ballast resistor.

More details on the ECU response to the air temp sensor and head temp sensor are on my ECU web page:

http://members.rennlist.com/pbanders/ecu.htm

Better ways of affecting the overall (idle/part/full-load) mixture are by varying the fuel pressure (no more than +/- 3 psig from the stock setting of 29.4 psig, more=rich, less=lean), and by adjusting the main mixture screw in the manifold pressure sensor. I suggest to anyone who is considering such adjustments that they do them ONLY after having the car characterized on a chassis dyno while hooked up to a shop quality exhaust gas analyzer. All adjustments should be done on the dyno, too. Blindly making these adjustments without reference to power output and actual mixture data is a bad idea ."
Sorry for copying this without your permission! Kudo's for the great insights
Geoff

Great Stuff! Brad never ceases to amaze me with that site!

Good point that the ever changing head temp would mean an everchanging signal to the ECU, altered (with the non oem resistance) to look like a warm up resistance, cool down, warmed up, ....

Maybe a circuit that ran to something other than the CHT to remove the fluctuations in head temp? It would work like the old days when everyone got a manual choke. Warm it up and set it?

Dash mount Air/Fuel ratio guage to set it?

Thanks,

....Doug

"Dash mount Air/Fuel ratio guage to set it?"

Not accurate enough for that, good to watch as you drive down the road.
The way to do this is to go to a dyno and adjust your MPS which is similar to what Dhunt did the other day with his Megasquirt FI.
Read the data on Brads site about adjusting the MPS. Its very easy with the right tools.
Geoff

Hi Mike,

Thanks for the encouragement!

No worries, I'll get it tuned yet, just don't want to mess with the MPS unless I absolutely have no choice. Brad himself warns against it. Besides, dyno tuning is for a setup that is "done", it is the final step and I'm not quite there.

Brad's point about the CHT sensor changing the warmed up mixture when balast resistance is added, got me to thinking (ouch). Why not bypass the CHT and go straight to ground with the negative side of the pot?

Well, I tried this today and it's better I have a potentiometer in place of the CHT and seems to work fairly well. Warn start seems rich unless I set the resistance to zero, other than that it is running richer across the range with no drivability problems.

I will add a Air/Fuel gauge today, just to have a better idea of what's going on even though it is no replacement for the real equipment.

Need to get this sorted, Thursday I'll be at the California Fest of Speed!

Thanks to Geoff for his help

....Doug