D-jet MAP sensor

[bill]

Anyone know if a D-jet MAP sensor can be adjusted? How? It's been rebuilt by Fuel Injection Corp (who suggest sending it back for "adjustment" since my coupe still won't run...) I'll send it back if I have to...trying to avoid another month's delay getting my coupe back on the road.) Thanks for any help!

 

[sfdon]

Why not install a known working unit to prove whether yours is the culprit?

 

[bill]

I'm hoping someone in NJ/Pa. will make that offer...

 

[sfdon]

The E9 board though the generosity of its members offers a lending library of known good d-jet components to members seeking to diagnose engine problems.
You pay shipping to and return is the only cost.

 

[Markos]

The E9 board though the generosity of its members offers a lending library of known good d-jet components to members seeking to diagnose engine problems.
You pay shipping to and return is the only cost.

Ohh we need a lending library forum. One thread per item! Track recipients and publicly shame them when overdue.

I’ve also offered quite rare temporary parts to trusted recipients, but nobody has taken me up on them yet.

I contribute a rear fog light template (done and CSI airbox bracket template (wip).

Anybody have an iron maiden?

 

[tochi]

Anyone know if a D-jet MAP sensor can be adjusted? How? It's been rebuilt by Fuel Injection Corp (who suggest sending it back for "adjustment" since my coupe still won't run...) I'll send it back if I have to...trying to avoid another month's delay getting my coupe back on the road.) Thanks for any help!

I am no expert on this subject.

In answer to your first question, yes, the sensor can be adjusted. However, blind adjustment without knowledge, experience or dumb luck is probably not the best approach to your current problem. In other words, swapping out your questionable unit with a known properly functioning device makes the most sense. Of course, availability makes this easier said than done.

Generally speaking, as described below, you can bench test the device for its ability to hold a vacuum and for continuity.

Most of the answers to your second question, "how" appear to be covered here >>
https://members.rennlist.com/pbanders/manifold_pressure_sensor.htm

Caveat, proceed at your own risk.

MPS Diagnostics​

Before proceeding on any calibration or adjustments of the MPS, it is essential to check the basic operation of the unit:

• Using a hand vacuum pump with a gauge (e.g., Actron model from Checker, about $30), pull a vacuum of 10 in. Hg on the sensing port of the MPS. If a vacuum cannot be pulled or it leaks down (< 5 in. Hg in one minute), then the sensor is leaky and must be repaired. The usual causes of vacuum leaks are a cracked diaphragm, a leaky body flange seal (either an o-ring and/or a flange seal), or a leaky part-load stop seal (paper seal, unobtainable - gasket sealant may work). Some people have reported luck with repairing the diaphragm using solder or epoxy, or a sandwich of aluminum foil and RTV (!), but such a repair is not recommended. The diaphragm is fully flexed each time the engine is under full load (open throttle) and it is highly likely that any repair will only hold for a short while. The usual way of getting a repair is to do a rebuild exchange with either Performance/Automotion or Automobile Atlanta.
• If the vacuum test is passed, check the coil resistances. The primary coil is measured between terminals 7 and 15 and should measure about 90 ohms. The secondary coil is measured between terminals 8 and 10 and should measure about 350 ohms. Also check the coil isolation - there should be no continuity between either of the coils and the case. If any faults are detected the MPS is defective and must be replaced.
• If both tests above are passed, then the MPS should be in good condition. However, it is possible that one or both of the aneroid cells have failed and are no longer sealed. This failure causes a rich part-load and idle mixture. Testing for a failed aneroid cell requires measurement of the inductance vs. vacuum characteristic of the MPS. The recommended inductance meter for this test is the Wavetek LCR55 (~$175) . The slope of the part-load characteristic is compared to the calibration curves presented in this document. If the slope is significantly (~50%) less than the calibration curve, then it is likely that one of the cells has failed.

Most - but not all - MPS sensor adjustment points are sealed to deter tampering or adjustments. Removal of the epoxy without damaging the unit can be a significant chore.

Adjustments and Effects on Mixture​

We now discuss the adjustment of the inner and outer screws, full-load stop, and the effects on part-load and full-load operation. We'll start with a model MPS that is properly adjusted for part-load and full-load operation. "Tighten" = clockwise, "Loosen" = counterclockwise.

Inner Screw​

If we tighten 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 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.

Outer Screw​

Under part-load conditions, the bottom of the outer screw is held against the part-load stop by the pressure differential across the diaphragm. If we tighten the outer screw independently of the inner screw (meaning we prevent the inner screw from turning with respect to the body of the MPS), the middle of the diaphragm is moved away from the part-load stop, decreasing its throw, and decreasing the amount of enrichment under full-load operation. Additionally, tightening the outer screw independently of the inner screw has the same effect on part-load operation as loosening the inner screw, enriching the part-load mixture.
Similarly, loosening the outer screw independently of the inner screw moves the diaphragm towards the part-load stop (note: in the condition I'm describing here, the MPS is under part-load pressure, and the bottom of the outer screw is in contact with the part-load stop plate), increasing the throw of the diaphragm, resulting in a larger degree of enrichment under full-load operation. Also, the effect on the inner screw is the same as if it were tightened, leaning out the part-load mixture.
However, if the outer and inner screws are "coupled" and the inner screw is turned the same amount as the outer screw, then the part-load mixture adjustment is unaffected, and the effect on the full-load mixture is the same as described above. This behavior will be exploited in describing how to properly adjust the MPS.

Full-Load Stop​

The full-load stop adjusts the maximum movement of the diaphragm. Once the diaphragm engages the full-load stop, if the pressure increases further in the MPS, the mixture will continue to be richened, but at a rate determined by the expansion of the aneroid cells and not by the movement of the full-load diaphragm. Note that if the full-load stop is adjusted so that it does not make contact with the diaphragm under full-load conditions, the diaphragm will be unsupported. This over-stresses the diaphragm near the flange and leads to early failure. Most MPS's are adjusted so that the vacuum level where the full-load stop is engaged is about 2 in. Hg below atmospheric pressure, though I have seen some that were adjusted to 4 in. Hg. Note that at high engine speeds, due to the pumping restrictions of the intake, even with wide-open-throttle the intake manifold is about 1 in. Hg below atmospheric pressure.

Coil Spring​

While this is not an adjustment that can be made on a specific MPS, the spring constant of the coil spring sets the vacuum level where the onset of movement of the diaphragm begins (Po). Bosch employed at least two different types of coil springs in the MPS's used with 914 applications that gave different values of Po. Units with the different coil springs can be identified by "short" and "long" coil spring holders in the main casting of the MPS. As mentioned above, the "short" coil spring units have a Po of about 6 in. Hg, and the "long" units have a Po of about 8 in. Hg (based on a very limited sample).

Adjustment effects are summarized in the table below:

Adjuster	Adjustment	Part-Load Mixture	Full-Load Mixture
Inner Screw Alone	Tighten	Leaner	Leaner
	Loosen	Richer	Richer
Inner & Outer Screw Together	Tighten	No effect	Leaner
	Loosen	No effect	Richer
Full-Load Stop	Tighten	No effect	Leaner
	Loosen	No effect	Richer

 

[Arde]

Ohh we need a lending library forum. One thread per item! Track recipients and publicly shame them when overdue.

I’ve also offered quite rare temporary parts to trusted recipients, but nobody has taken me up on them yet.

I contribute a rear fog light template (done and CSI airbox bracket template (wip).

Anybody have an iron maiden?

Is there a known good M90 engine in the library? I would love to try it.

 

[bill]

Tochi: unbelievable info! Now I understand the difficulty of adjusting the MAP. This is great, thanks for posting.

 

[bill]

The E9 board though the generosity of its members offers a lending library of known good d-jet components to members seeking to diagnose engine problems.
You pay shipping to and return is the only cost.

Thank you SF Don. PM sent.

 

[sfdon]

i will scrounge around in shop for trigger and MPS today and Stephanie will ship tomorrow- I’m leaving in am to drive the Twisted Sisters in Texas next week so nothing else til the 9th of August.

 

[bill]

i will scrounge around in shop for trigger and MPS today and Stephanie will ship tomorrow- I’m leaving in am to drive the Twisted Sisters in Texas next week so nothing else til the 9th of August.

Thank you Don!

 

[bill]

I am no expert on this subject.

In answer to your first question, yes, the sensor can be adjusted. However, blind adjustment without knowledge, experience or dumb luck is probably not the best approach to your current problem. In other words, swapping out your questionable unit with a known properly functioning device makes the most sense. Of course, availability makes this easier said than done.

Generally speaking, as described below, you can bench test the device for its ability to hold a vacuum and for continuity.

Most of the answers to your second question, "how" appear to be covered here >>
https://members.rennlist.com/pbanders/manifold_pressure_sensor.htm

Caveat, proceed at your own risk.





Most - but not all - MPS sensor adjustment points are sealed to deter tampering or adjustments. Removal of the epoxy without damaging the unit can be a significant chore.

Barely into the article and I find the idle vacuum is supposed to be 15 in/Hg. My MAP, rebuilt by Fuel Injection Corp. is set at 18"/Hg. This could be why my engine runs like it's stuck on full choke (too much vacuum is confusing the ECU.) I hope I'm on to something. Thanks again Tochi.

 

[sfdon]

Supposed to be ABOVE 15 for idle ?
What is your current vacuum at idle?

 

[sfdon]

Too much vacuum at idle is not a problem

 

[bill]

FI Corp. says they set my rebuilt MAP to 18"/Hg. After explaining my problem with very poor running, they thought "maybe the idle setting was too high" (too much vacuum,) and suggested sending it back to them and they will re-adjust. Hoping that works, but it seems like a "hit or miss" way to make an adjustment.

 

[sfdon]

You need to answer the question-

What is your current vacuum at idle?

 

[sfdon]

You have perhaps confused your poor running and low idle vacuum with a problem with the MPS. If your engine is running poorly and vacuum is low at idle- the system won’t work.

 

[sfdon]

You need to call me- you have the vacuum info backwards.