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  #1  
Old 09-11-2009, 11:52 AM
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Adjusting MPS on D-Jet

I have a CO meter on loan and wish to check and adjust the emissions, now hat I have car running reasonably well.

What is recommended procedure?

Does MPS get adjusted at some higher rpm?

Idle screw on ECU - is that just fine tuning after MPS is set?

Tried to find this on web, but my search skills must be lacking
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  #2  
Old 09-11-2009, 12:18 PM
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Hi,

The screw on the ECU is to set the mixture at idle only, make sure your timing and idle speed are within spec before twiddling.
Unless your emissions are way off at non-idle and you've eliminated all other causes I'd personally leave the MPS alone as you could end up chasing your tail.
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Old 09-11-2009, 01:03 PM
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Quote:
Originally Posted by GaryF View Post
Hi,

The screw on the ECU is to set the mixture at idle only, make sure your timing and idle speed are within spec before twiddling.
Unless your emissions are way off at non-idle and you've eliminated all other causes I'd personally leave the MPS alone as you could end up chasing your tail.
I understand your concerns!

But, in order to improve hot-starts, I have set fuel pressure slightly higher than spec - 30-32psig. Even higher could be better. This would result in richer mixture, so I want to check and make small adjustment if need be.

At what rpm should I first be checking CO? i.e what is non-idle?
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Old 09-11-2009, 02:07 PM
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IIRC,the CO setting is checked at idle,around 700-800RPM
Make the adjustments in small incriments,no more than 1/4 turn at a time as a little bit goes a long way!
It would probably be a good idea to "center" the fine tuning knob on the ECU before starting any adjustment.
My car was WAAAAAY rich and we got the CO to about 0.35% by adjusting the MAP.Runs much better,better throttle response(my main complaint),and about 2-3 MPG better as well.
The CO varies by model year,you will need to check your under hood label to be sure.Mine (1974)was 1% or below.
Before getting access to the CO meter,I tried to adjust by using vacuum gauge to get highest idle vacuum.This did not work at all as highest vacuum resulted in very rich mixture!
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Old 09-11-2009, 03:24 PM
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I have been trying to adjust.

ECU knob at mid range. I am getting readings in the 3.5-6% range. Even making 1/2 a turn doesn't seem to help.

Meter may not be 100% and it does drift off it's balance mark and I have to reset it. (It's a Hawk Air/Fuel & %CO meter)

I do hear a change in idle speed depending on which way I turn it. Same with ECU knob.

Dumb question #1: I read that turning MPS screw counterclockwise leans the mixture. But is that when looking at screw from engine side or when looking at screw from fender side

Dumb question #2: If engine speeds up when MPS or ECU screw is turned, is it getting leaner or richer?

Just to be sure
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Old 09-11-2009, 09:51 PM
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Update on above.

I checked trigger points and they are once again malfunctioning. When car is cold they seem to work, but when I hear the stumbling at idle when hot, I find that at least one set of contacts is not opening for the same period as the others.

I have two sets, so will try and tweak one set so they work! Must be that the sliders are worn after all these years and don't push the points open sufficiently - No easy way to check what they are doing either unless I cut one distributor in half
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  #7  
Old 09-12-2009, 12:52 AM
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Map sensor, unscrew = cclw (when facing screw) will lean the whole system all rpm ranges, This device senses the difference in air pressure/vacuum. I had real good luck adjusting it driving and checking spark plug color.Try running the car 10 miles, check with CO meter, adjust 1/4 turn lean, run 10 miles test with CO meter. The thing I like about the Djet is its adjustability to compensate for aging and variations in motor compression decline.
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Old 09-12-2009, 07:11 AM
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The MAP and ECU screws are opposite.

Turn the MAP CCW to richen, CW to lean.
Turn the ECU screw CCW to lean, CW to richen.

The ECU screw won't have any effect on idle mix unless your TPIS is adjusted right.
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  #9  
Old 09-12-2009, 08:37 AM
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according to my books - test CO at 700-800 .5-2.0 %
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Old 09-12-2009, 09:29 AM
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Seems to me that adjusting the mixture by way of fuel pressure would be a bit imprecise as compared to adjusting the MAP sensor, no? But maybe that's the only way to improve hot start problems.
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  #11  
Old 09-12-2009, 10:00 AM
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Quote:
Originally Posted by Tomguy View Post
The MAP and ECU screws are opposite.

Turn the MAP CCW to richen, CW to lean.
Turn the ECU screw CCW to lean, CW to richen.

The ECU screw won't have any effect on idle mix unless your TPIS is adjusted right.
Quote:
Originally Posted by BWOSTOSH
Map sensor, unscrew = cclw (when facing screw) will lean the whole system all rpm ranges
Maybe it depends on how we look at the MPS screw, but don't the above posts contradict each other?

By the way Tomguy - I have carefully adjusted the throttle idle switch. I had not realized how sensitive this is. I suspect many D-Jets are running without the idle switch closing. At idle, the injectors see vacuum on one side, so differential is something like 39 psig while when throttle plate is open differential is 30 psig. Presumably ECU adjusts injector pulses to account for this?
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  #12  
Old 09-12-2009, 05:09 PM
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This article explains in great detail how the MPS functions and how to properly adjust it.

http://members.rennlist.com/pbanders/manifold_pressure_sensor.htm
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  #13  
Old 09-12-2009, 06:03 PM
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Quote:
Originally Posted by Tony H View Post
This article explains in great detail how the MPS functions and how to properly adjust it.

http://members.rennlist.com/pbanders/manifold_pressure_sensor.htm
You have to dig deep on that site to get to the part that counts The screw we are turning is presumably the "inner screw" and hopefully the outer screw is not moving.

This is what the site says (my additions in parentheses)

Quote:
"Tighten" = clockwise, "Loosen" = counterclockwise.

Inner Screw

If we tighten (turn CW) the inner screw independently of the outer screw (meaning we don't allow the outer screw to turn with respect to the body of the MPS as we turn the inner screw), the armature is driven out of the coil, reducing the inductive coupling, causing the ECU to shorten the injection pulse. This adjustment will make the mixture leaner across both part-load and full-load, because the inner screw is directly coupled to the full-load diaphragm.

Similarly, loosening (turn CCW) the inner screw independently of the outer screw moves the armature into the coil, lengthening the pulse, and enriching the mixture across both part and full-load.
If this article is correct for MPS

- Screw in; tighten; turn CW results in LEANER mixture
- Screw out; loosen; turn CCW results in RICHER mixture

For the ECU, shop manual says
- turn CCW for leaner mixture
- turn CW for richer mixture

Does everyone agree??
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Old 09-13-2009, 04:44 AM
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No. Adjusting the MPS is not the way to tune a Djet. Running excess pressure doesn't improve anything either.
The injectors are designed to run at 30 PSI max. Upping the pressure prevents the pintle from closing leading to dribbling and bad running.
Your running fualt sounds to me like the air temp sensor and possibly the water sensor are sick.
Instead of checking those you have upped the pressure exacerbating the problem.
Trigger points will last for years if kept clean. If the distributor is worn,the shaft wobbling oil leaks up into the points and insulates the contact surfaces.
have you done the basics? Checked timing chain,valve clearance,igntion? all have to be perfect before messing with the injection.
Have you checked the return line from the engine to the tank to ensure there isn't any obstruction?
Have you done a flow test on the injectors and checked the cold start injector to make sure it isn't dribbling?
Is the TPS clicking evenly across the range?
Are all the earth connection points clean ?
Is the line to MAP sensor secure?
Is the distributor centrifugal advance working?
A djet should fire as soon as the key is turned .If it doesn't,there is a fault that must be rectified.
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Old 09-13-2009, 10:36 AM
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Quote:
Originally Posted by mercmad6.3 View Post
No. Adjusting the MPS is not the way to tune a Djet. Running excess pressure doesn't improve anything either.
The injectors are designed to run at 30 PSI max. Upping the pressure prevents the pintle from closing leading to dribbling and bad running.
Your running fualt sounds to me like the air temp sensor and possibly the water sensor are sick.
Instead of checking those you have upped the pressure exacerbating the problem.
Trigger points will last for years if kept clean. If the distributor is worn,the shaft wobbling oil leaks up into the points and insulates the contact surfaces.
have you done the basics? Checked timing chain,valve clearance,igntion? all have to be perfect before messing with the injection.
Have you checked the return line from the engine to the tank to ensure there isn't any obstruction?
Have you done a flow test on the injectors and checked the cold start injector to make sure it isn't dribbling?
Is the TPS clicking evenly across the range?
Are all the earth connection points clean ?
Is the line to MAP sensor secure?
Is the distributor centrifugal advance working?
A djet should fire as soon as the key is turned .If it doesn't,there is a fault that must be rectified.
You may not have read all my posts on my current problem.

My fuel pressure was once low - 27psig. I did bump it up. MB say range can be 2- 2.2bar (31.9psig). Some very knowledgeable MB mechanics say they have used up to 35 psig.

As I understand it, these injectors are flow rated at 3bar (43.5psig) but are used at various pressures depending on the flow requirements of the particular application. At idle with 20"vac, the injectors see 10psig greater pressure differential than when throttle is open. So we have 30psi differential with throttle open and 40psi when idling. Injectors must be able to handle this and more. Car will run rich if pressure is higher than design, and that is one reason MPS needs adjusting to bring CO back within spec.

Another factor: These cars were designed when fuels were different, at least here in North America. Current fuels are more volatile due to Ethanol addition and due to other additives put into winter fuel. I think I once found that winter fuel at high end of volatility spec would begin to boil at 27 psig and 100C. Modern cars that run with higher pressures, don't seem to have these problems.

Regarding my car. I have done just about everything you mention:

- Timing chain replaced last year
- Valves adjusted last year
- Compression tested (150psi across board last week)
- Every sensor recently checked both at sensor and from ECU plug - no faults
- Fuel pressure gauge on dash checked with bourdon gauge - 30psi at idle, 32 when running. Injectors hold pressure longer than MB specs.
- New plugs,,rotor, cap recently. Has Pertronix. 1 yr old wires Coil still original. New 0.6ohm resistor
- Injectors cleaned and flow tested.
- TPS removed, cleaned, tested and set so idle switch works.
- Ground connections cleaned
- Vacuum lines checked. Car runs at 16-19.5" Hg depending on timing
- Distributor dismantled and centrifugal weights freed up a year ago Recently confirmed advance with timing light.
- Cold start injector does not leak down. For testing I have it disabled.

Some of this was done a year or so ago and car has run well until recently. Current problem occurred about 3 weeks ago and started as a slight miss/stumble. That is when new plugs, rotor, cap were installed with no improvement.

I am now trying to get CO in spec, but problem, which seems to be injectors, is making that difficult.

Trigger points don't have oil on them and there is no sign of oil. But they don't open and close consistently. I have to eliminate this problem before I proceed further.
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Last edited by Graham; 09-13-2009 at 11:52 AM.
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Old 09-13-2009, 10:36 AM
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