MAP adjuster for cold, better mileage?

Is this a good idea? I've heard that when an XJ is cold it uses the MAP sensor to guage fuel delivery, then switches to using the O2 sensor when it's warmed up. My problem is that I live in a cold climate and even with my grill mostly blocked it takes a while to warm up to the warm cycle. Since the warm up cycle normally runs richer than "operational temperature" cycle I see a definate impact on my mileage. I loose easily more than 15% gas mileage when it's cold out. This is with exactly the same driving/trips etc.

My question is, can I use a MAP adjuster to lean out the warm up cycle to get better mileage? And assuming I calibrate it rich enough to keep the engine from stumbling, are there any other side effects I can expect from running leaner? Lastly has anyone tried this and if it works, what voltage works best for you?

Also I run a home made snorkle to get more air, and colder air. Would I get much benifit from blocking it?

92 XJ Loredo 4.0

I have a better idea. Get a Robertshaw 205* t'stat from http://www.flowkooler.com/thermos.php?FKSID=187af7d07d79afee4c0009705466a029 . This will allow your engine to warm up faster by keeping the t'stat closed for longer.

What you heard is not exactly correct - the MAP sensor & the O2 sensor are both used in the cold start cycle and most other times while in operation. No matter what you do it will take X amount of time to heat 500 lbs of metal and coolant from 0 degrees to operating temp and that is maybe double the time it takes from 90 degrees to op temp. blocking the radiator will do little, removing the fan would help more but has its own drawbacks. Cold weather driving is going to use more fuel no matter what you do.

Dr. Dyno said:

I have a better idea. Get a Robertshaw 205* t'stat from http://www.flowkooler.com/thermos.php?FKSID=187af7d07d79afee4c0009705466a029 . This will allow your engine to warm up faster by keeping the t'stat closed for longer.

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I don't see how this would make it warm up faster, the warm cycle should already be in use when a OEM termostat opens no? It would make it run at a higher temp once it's warmed up yes, but I don't think that is where my fuel economy prob comes from.

rsalemi said:

What you heard is not exactly correct - the MAP sensor & the O2 sensor are both used in the cold start cycle and most other times while in operation. No matter what you do it will take X amount of time to heat 500 lbs of metal and coolant from 0 degrees to operating temp and that is maybe double the time it takes from 90 degrees to op temp. blocking the radiator will do little, removing the fan would help more but has its own drawbacks. Cold weather driving is going to use more fuel no matter what you do.

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I understand to heat the engine takes more heat when it is colder, what I'm trying to accomplish is to cut down on the extra fuel consumption when the engine is cold, even if that means the engine takes longer to warm up. Frankly when i'm driving in down to -45' temperature the engine doesn't ever warm up to "normal operating temperature" anyways. But it does burn more fuel, and I can smell the rich fuel in the exhaust.

I have been tracking my mileage for years with several vehicles but none of them lost as much fuel economy in the winter as my baby jeep. That includes some cars that needed to be left ideling in the driveway before they would run. The jeep starts in the cold, runs smooth and warms the interior to a comfortable level quick without leaving it running in the driveway frist.

Anyway, for the cost of building a map adjuster I'm going to try it and let you jeepers know what I find.

Beached Bones said:

I don't see how this would make it warm up faster, the warm cycle should already be in use when a OEM thermostat opens no? It would make it run at a higher temp once it's warmed up yes, but I don't think that is where my fuel economy prob comes from.

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It might at least partly help. If nothing else, your heater will work better in the cold winter months so it's worth the $10 it costs for the t'stat.
Substituting the clutch fan for an electric fan will certainly help the engine warm up faster because you won't have a mechanical fan trying to cool the engine when you want it to warm up.
You'll cut down the fuel consumption during the warm up cycle by driving straight off from a cold start rather than leaving the engine to idle for a few minutes.

Update

Map adjuster is built and installed, so the next tank of gas may tell me something. For once in my life I want cold weather, but it's warmed up here this week.... figures.

I had a little trouble finding a consistant 12v switched power point to draw from. Right now I'm tapped into ignition power right off the PDC. I tried Cig lighter first but it occasionally loses voltage at starup. Anyone know of a better place to get a stable 12v?

Beached Bones said:

Anyone know of a better place to get a stable 12v?

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You can use the 12v power feed for the electric fan.

Results from test

My overall average mileage since i bought my xj is 14.823 MPG (over 84 tanks of gas, best mileage was 21.652 MPG worst was 9.263 ) Stock MAP voltage measured at 5.05v

Over the last 5 fills before installing the map adjuster I averaged 13.782 MPG

Map set at 4.90v gave 15.027 MPG
Map set at 4.80v over 2 tanks gave 12.675 MPG

At 4.8v my xj was starting to feal a little ruff. at 4.9v seemed to run smooth

I'm disconnecting my MAP adjuster for the next tank to see if I return to normal mileage. After that I'll adjust my home made snorkle to suck warm underhood air rather than cold outside air.