The MAP sensor in a MAF engine is used to sense atmospheric pressure.

 

I have not seen a model that used both a MAF and a MAP simultateously. A MAF sensor measures air density, so I would see no point in using a MAP as well. I know that my '94 4.0L does not have a MAP sensor. Sam with my parents 3.0L which is a '92

 

My 90 and 91 both have MAF and MAP sensors, but the port that is normally routed to the intake manifold is just vented to the atmosphere, where it's measuring the ambient pressure. If I apply a vacuum to it, I can see minor fluxuations in the engine speed, so it's doing something.

 

This is an interesting thread, please forgive my jacking for a moment. My '95 4 L AWD XLT has a similar issue that recently developed. When the engine is cold the engine runs very rough as if it's dropped a cylinder. Usually by the time I've put a mile or two on it and there's a little engine temp, the engine smooths out and all is well. I've had the vehicle for a decade and it has always acted up at elevation. There it loose power, knocks like crazy and kicks on the CEL. This engine too has new(ish) plugs, cats and o2 sensors. I'm intrigued by the idea that a MAP/MAF fault might cause all this. Does this seem reasonable?

 

Ok, I think we are seeing a pattern here. Your '90 and '91 have both, by my parents '92 does not. Behaps '91 was the last year to use both sensors.

 

BCMike
your '95 uses a MAF to measure engine air intake amount.

have you ever cleaned and inspected the the MAF and cleaned the TB? poor high elevation operation is indicative of dirty contaminated MAF sensor wire element

poor cold operation can also be caused by a failed ECT sensor, 2 wire on front top of engine to driver side of thermostat housing

 

Cheers 96_4wdr,

I just did a trip with the MAF unplugged to run the open loop and had no missing or hesitation issues, but I didn't have it under load or at elevation and it was the second run of the day. I did have the MAF replaced when trying to deal with a NOx issue and AirCare but I haven't cleaned it ever. I will pull it and give it a good dousing with electrical contact cleaner. I'll follow up on the ECT sensor too. I really appreciate the tips!

-Mike

 

Having a MAF sensor on your vehicle means that it will be used for fuel/air control. If you have an MAP sensor it will be used for Atmospheric air pressure (altitude & air density adjustments only). SOME vehicles use MAP for fuel/air control but not 96 on Aero's.

 

There are a lot of conflicting statements & ideas in this thread. It is really very confused & confusing. So here's a simplified lay version "EFI intro 101"

MAP sensors are integral part of Speed density logic, EFI systems, used by the ECM to figure out what RAM program to apply. One reason Speed Density Programs lack broad performance range, fast duty cycles & host of fast generated, RAM functions, is they are both slow and narrow, unlike newer Mass Air logic based systems.

Original EFI ran by Speed Density Logic which uses set parameters & fixed limitations.
When they are violated, exceeded or unachieved systems would fault out & run badly.
Manifold Absolute Pressures are a relatively limited or range based set of values. MAP was well suited to the limited, early EFI Speed desnity Programming. But in '92, a new broad based, fast operating, wide RAM logic, MAFS system appeared on the market.

When EEC IV systems went to Mass Air Flow Sensed logic, in lieu of old Speed Density
logic, MAP (Manifold Absolute Pressure) had no purpose. However, BAP is needed to get a baseline of Barometric Absolute Pressure, to form comparitive analysis needed for new MAFS or "mass air" algorhythyms to be accurately functional. It's what both of the flow volume calculations [air & fuel] of the MAFS ECM are based on.

Volume of flow is what determines fuel quantity required to do whatever is needed as determined by other sensors in a system, aka TPS, O2, Speed, RPM, Ign timing, Spark knock, Octane value, etc. et al. System uses Brake Specific Fuel Consumption or BSFC specific algorhythyms to optimize fuel use to the best or higest advantage standard.

As mentioned above MAF or MAFS measures Air Flow which by combining BAP & Speed, the system comes up with an averaged volume & density of air being taken in by the system and on that determines fuel volume & timing requirements.

With no idea of ambient atmosphere or Barometric Absolute Pressure,[infinitely variable value] operating system has no clue as to elevation or changing relative atmospheric pressures from which it is drawing Air into intake and is measuring flow of. Lacking that data, the ECM can not make the "Either/Or" decisions it's greater logic [than Speed Density has] for making those determinations. But generally the Mass Air based system operates @ 300 times faster, plus has virtually unlimited capacity to generate modified RAM instructions to an O/S, or operating system

People in the trade tend to homogenize or commonize names over time. For example people commonly call the Ignition locking cylinder an Ignition switch, "It Ain't a Switch" however, it's a lock. I'm sure they do the same with MAP & BAP since MAP came 1st. Even though the technology has moved on, their language has not. . . .

In short the entire EFI sensor System all works to control fuel flow, but our foot on the throttle pedal is what controls the Air flow, and it does it by using the tps. The only sensor with moving parts in an EFI system. . . .

Whew! I hope I got that right

FBp

 

Excellent summary, and I think that it clarified a lot.

On the engine running rough, my MAF failed recently. When it did, I experienced a major loss of power and the engine as could be expected, ran very rough. However, in my case, a CEL was triggered. Just prior to the light coming on, I noticed a lag in the throttle response, which seemed abnormal.

 

Bear River

If Air flow volume is not measured, or does not comply with what is possible [it's really wacky or no values] coming from MAFS because it's faulty, your ECM should default to demands until it can act in a manner so as to not damage rest of the system. Sort of a modified "limp in" alternative to a catastrophic action. Suppose when you tried to accelerate, the sudden but "unread " condition resulted in a "back fire". . . ?

We know backfires in EFI intakes can wreak havoc to systems. For 1 thing all vacuum lines without backfire protection valves get a blast of reverse uncontrolled positive + pressure instead of vacuum or neg' pressure. Diaphrams rupture & distort etc. The Air Duct can blow off the throttle body, weak vacuum lines can be blown off, sensing wire in MAFS can be blown south, all sorts of gremlins can show up.

So what felt "abnormal" according to "normal" operation criteria is normal in a default mode. The breadth of the superior Mass Air Logic allows that to occur & so qwikly too. The CEL appeared then when ability to function relatively normal was negated during a search for a alternative response.Your CEL let you know it had problems. . . . Nothing more than what I'd expect from a FoMoCo system, myself. AND your system didn't destroy itself!

A speed density system would backfire because it has nothing else it could do. It's too small, slow a P-ROM program. Also, it has to "Loop" or Bank Fire, before it does anything. Plus S/Ds can't extend or extraoplate to alternative or default actions without going full sequence or "bank". So your "ABNORMAL" was really normal, given problem you had developed, ergo faulty MAFS. But operational Mass Air Based system worked as designed & saved you $$ too.

FBp

 

okay back to autozone to swap map for maf. Drove it down the street and back with hutch off and I heard a whistle when the engine is under the most strain. I'm at a complete loss - finding someone to work on a vehicle around here without letting it sit in their lot for months while they get around to it is very difficult and the ones I do find won't touch it.

 

whistle while engine under load indicates an intake air leak somewhere from air filter to heads or in vacuum lines...common with Aero 4Ls

could also be exhaust leak but they usually have a varying pulsing or popping sound with rpm change

 

Sounds like ghenryl's problem...like unmetered air is getting into the engine. See if there is a loose vacuum line. That is very likely if you had a backfire.

Also, when EEC systems first came out, the MAP sensor was actually two sensors in one housing; one for the manifold and the other for the atmosphere. Obviously, the atmosphere pressure sensor is for compensation of high altitude (low pressure) operation. And it's interesting that the racers prefer speed-density over MAF because the MAF sensors tend to form a restriction in the air path, and their responses are a little slower than the SD systems. The MAF measures air mass, which is in theory more accurate than inferring it from speed density. But it does it at a location a few feet ahead of the cylinder, so timing is more critical than SD, which measures the pressure in the manifold, much closer to the cylinder.

The other problem with Ford's MAF sensor is that it's a bypass type air flow device. The sensing element is not in the primary air path; it sits in a small side path, and the whole process relies on the calibrated pressure differences between the main and side paths over the range of designed flow rates. As long as the air flow rates remain in that range, the measurements are very accurate. The hot rodders get into trouble when they start going out of that range, and try to compensate by trying to fool the sensor.

It's much easier to get into trouble with the SD system when the engine is modified, as it has to infer the air mass from measurements that no longer represent the conditions of the original engine. But the SD system, like the MAF system, can be retuned to work well with modified engines.

 

My van too falls into this problem arena...thus the reason I'm back here looking for help. I have a 95 Aero with the 4L which is having a problem similar if not identical to yours. I would check for a vacuum leak. One of my problems is that I continue to have vacuum lines pop off when I go to start the van and it backfires when it doesn't start. There is a (what I call a vacuum tree) vacuum post located on the top/back/drivers side of the engine. There are 4 or 5 vacuum lines going to it. They like to pop off and cause the engine to idle and run rough. I've had to put cable ties on a couple of them to keep them on. I only have one that is still giving me fits staying on. I tried Saturday to put a cable tie on it but it won't fit with the other hoses and ties. Some I had to reach from the inside, others from the outside, and some I needed help from the inside while I reached in from the front. I can't see the vacuum tree so I have to feel around and see if there is a hose off and then feel around to find it. Good luck!

I took my MAF sensor off yesterday and cleaned it. I also took the battery cable off to reset the computer. I drove it down the street a ways but it is still running rough. I bought plugs for this weekend but I'm not looking forward to replacing them. I will try to replace the rotor, cap and plugs while I'm in there getting scratched up by the engine. I also bought the front O2 sensor last night (after reading this post) and will replace it first. I will stay tuned to this thread to see how your problems go and post what I do to fix mine.

I also had a problem to where my transmission modulator went out. I had zero fluid on my trannie dip stick and 5" of fluid up my oil dip stick. I'm having to deal with oil soaked vacuum lines as well. Anything I can do to fix this?

Thanks for your insight.
Reid
Spokane, WA