DME chip questions
 

It has been suggested that a DME chip, due to it's mapping, would increase my "running" mixture to a richer condition. My car, as most of you know, has been running lean, regardless of my efforts to correct it. It idles fine but stumbles a little during normal acceleration.

Any comments or recommendations on the accuracy of this opinion and a possible product?

it all depends on the chip. some chip tuners use fuel. some use timing. some use both. if your mix is lean, and you use one that uses timing, you could get into real trouble.

if your car is running lean, i would resolve the issue before installing any chip. a chip is not the way to solve that problem.

also, how do you know it's running lean? have you stuck a wide band sensor into the pipe up front? (generally readings at the tail pipe are about a half point leaner than up front at the O2 sensor)

things that make a car run lean:
vacuum leak
exhaust leak
weak fuel pump
weak fuel pressure regulator
incorrect valve timing
clogged or dirty injectors
fouled or tired O2 sensor
bad or dirty MAF (if you have one)

definitely go down that list first. vacuum leaks are the number one cause of running lean.

This issue has be on going and has been discussed on this and other websites.
TPS, AFM, DME have been swapped out. 1000 miles on plugs and they read lean. When O2 sensor is unplugged (forces rich mixture) car accelerates fine. New items: TPS, O2 sensor, CAT, silicone vacuum lines, three vacuum tests verified no leaks, total ignition system replacement, new intake manifold gaskets, throttle body shaft seals, Fuel pressure regulator, fuel pump, fuel filter, fuel strainer, no cracks in j boot nor fittings, rebuilt fuel injectors. I even tried readjusting fuel quality switch (FQS) in DME to adjust mixture, to no avail.

This is why I was exploring the possibility another poster had suggested of a DME chip that might solve it.

how are you testing the mixture? purely by looking at the plug? that is not a good test. for example, the pictures in the chilton manuals are for cars of the 70s, where cars ran in the upper 12s and low 13s. cars run a LOT leaner nowadays, spending most of their time in the mid 14s, and what would have shown as a lean plug before, is normal now.

what are your mixture readings? do it under load, at 3k, 4k, 5k, etc

you should be at 14.7 at idle and at cruise.

unfortunately, I don't have the equipment to do that test. I can't see taking it to the Dealership at $135 and hour. I'm not even sure they would know what to do to a 944 to adjust the mixture. It runs fine at idle and cruise. It's only when I try to accelerate that it shows up.

Did you check the engine coolant temperature sensor? It has a major effect on how rich or lean the engine runs. Also, clogged injectors can cause a lean condition.

yeah - i'm going to step out of this one. i am now quite certain that diagnostics are required at a shop. without a mixture reading, you have no way of knowing that it's lean. without a pressurization test, you have no way of knowing if you have a vacuum leak. you need better equipment to determine what is going on.

if you don't have the tools to do this, i think you need to drop the car off at a good shop. it's not rocket science, but you need the right diagnostic tools.

I did test the vacuum system using air pressure and soapy water 2 times and once using brake cleaner and propane. If there is a leak I can't imagine where it might be. I even took the intake manifold off again for access to things I couldn't easily reach. Puzzling. I do appreciate your insights, and observations. Thanks.

Which temp sensor are you referring to? I replaced the one on the front of the engine, the DME temp sensor. Where's the other one?

no worries. if you were able to pressurize the system, and hold pressure, then you have no leaks. if it didn't hold pressure, then there is a leak somewhere. i have a tool that clamps onto the front of the throttle body, and then i charge it with about 8psi (any more than that and you'll blow hoses off). i then use a gauge at the fuel pressure regulator hose to check. it will take a minute or so for the pressure to fall. if it falls right away, there is a leak.

you can also use a vacuum gauge during idle. is should be strong and steady, and about 17". if it's just a little bit lower (16 or so), then you have a small leak. if it's lower than that, you have a large one.

the other sensor should be right behind the front one

Now that's some interesting stuff....Really!. Where do you install a vacuum gauge for the idle check? FPR? Where do you find a vacuum pressure gauge?

Simple gauge available at Harbor Freight - vacuum tester.

yup - a simple tee at the line for the fuel pressure regulator, and tie the gauge in there for the reading

and here is a shot of the gadget i made up for the front, so i can pressurize the system:
http://forums.pelicanparts.com/uploa...1407416472.jpg

I like the fitting Flash. I have a few questions about it. I have a fitting that goes into the j boot that I can pressurize with air that I used for vacuum leaks. It worked fine but your's eliminates the j boot and goes right into the TB. Question 1. Where did you find that rubber boot? Question 2. How do you shut off the air source without losing the air charge? The gauge is an air pressure gauge, right?

On using a vacuum gauge for idle pressure, do you hook it up to the vacuum line on the FPR?

actually, mine goes onto the MAF. you can change the adaptor hose to whatever size you need to fit whichever component you want to clamp onto. the adaptor hose is from mesa hose. it cost as much as all of the other parts together.

the fitting on the left is a compressor valve (home depot). it has a schrader fitting at the end, which cuts off on its own. you pressurize with a standard tire valve gadget (or bicycle pump)

re: vacuum gauge. yes, you install a tee there at the FPR, reconnect the vacuum line on one end, a short hose on the other, and the third goes to the gauge.

i cannot stress enough the need to be able to control pressure, and not go over 8psi. failure to do that will result in blowing every fitting off, and possibly rupturing lines and hoses.

the other sensor should be right behind the front one[/QUOTE]

I checked the wiring diagram and found two temp sensors on the engine. One is the DME temp sensor, which is new and the other sensor goes to an overheat warning light and to the water temp gauge. The temp gauge is working fine and when I checked the DME temp sensor, The car wouldn't start without it installed. The ohms read about 2k ohms at ambient temp of about 85 f. After I got the engine up to 175 degrees temp (normal op temp) my ohm meter reads 00.5 in the 200 ohm range across the terminals on the sensor. So it looks like it's ok.

I've been seeing 10 psi at the tanks and 8 at then end of a 50ft hose so I'm covered. Thanks for the info on the setup.

I take it there's a giant cork plugging the exhaust pipe as well?

I suspect there may be an issue with the wiring between DME and O2-sensor plug:

1. replacement O2-sensor is good
2. unplugging O2-sensor makes stumbling issues go away.

Using a high-ohm meter, measure the resistance between the following terminals on the DME-plug (disconnected from DME). Voltmeter-ground on pin#24 and test resistance between:

1. 24 & 23
2. 24 & 16
3. 24 & 28

what should I expect to see on the above tests? I didn't suspect the muffler being an issue since I was able to see through the old cat and there was no under-carriage rust, and it performs very well at WOT.

Hey, Flash, where'd you get the air pressure gauge? All the one's I've found are in 20# increments up to 160 lbs. Air compressor gauges.