Goodo - we have a misfire and a problem with Kermit. It started mostly on the overrun, now it's reached the point whereby low load / low engine speed causes a kangaroo effect.
OBD-II reports a freeze frame fault code 0108, that my guide says, "Manifold Absolute Pressure / BARO Sensor High Input."
It's not something I can fix myself :) but I'm guessing that this is the funky little sensor that, heh, senses atmospheric pressure...?
Exactly what it says on the tin: It senses air pressure in the inlet manifold, and from that (along with air temperature and mass or air flow sensors) the ECU can work out the load on the engine and it's fuelling requirements - much the same way as you'd use a vacuum gauge for driving economically. If the sensor itself is kaput then it'll need replacing, however these sensors are usually mounted separate from the engine itself and connect to the inlet manifold via a small bore pipe. It would be worth checking this pipe and it's connections for leaks or obstructions before blaming the sensor itself. Also don't forget to check the electrical connections to the sensor - if one or more of them are bad or dirty (maybe due to water ingress?) then this could also register as a sensor fault...
Aye, and on the Fords they double up the purpose of the sensor, the MAP sensor measures air pressure before you start the engine, and then senses the air flow into the engine. If you travel up to any serious altitude in a Ford you do actually have to stop occasionally to let the enngine management recalibrate air pressure.
Bizarre. I know the original L-Jetronic had a mechanical air flow meter (or two in the case of the V6 engine), but later systems used a 'hot wire' mass sensor, temperature sensor and pressure sensor. Not sure how a pressure sensor could measure air flow - unless you mean the two sensors share the same physical housing and can't be replaced independently...?
Ford use two systems. The hotwire system is self calibrating, and does not need to measure atmospheric pressure anyway Ford call this MAF (Mass Air Flow). It measures the actual air flowing into the engine *before* the throttle butterfly.
Ford also use a sensor connected to manifold vacuum and call it MAP (Manifold Absolute Pressure).
This second system is not measuring how much is actually flowing, as with the hotwire system, it only measures the amount of pressure in the manifold *after* the throttle butterfly. As such it need a frame of reference to work from, and then it calculates the air going into the engine. When you initially switch the ignition on, it is measuring normal atmospheric pressure.
All of the Zetec engines use the hotwire MAF system, but AFAIK all the OHV engines use the MAP sensors. CVH engines use either.
One thing you can check is that the vacuum pipe to the MAP sensor hasn't got soft, and is collapsing under load. If it is, you need to replace it, and bung a standard one way valve in the line too.
Please explain the logic of the one-way valve - surely with one fitted it would prevent the sensor detecting changes in the inlet manifold air pressure (either negative or positive depending on which way round the valve were fitted)???!
The TMAP sensor appears to be producing duff information. When driving along the manifold pressure goes from normal up to full on (107, heh I don't know what the scale is) and then back down again. This causes one hell of a surge from the engine... repeat. Lots. The car is almost undriveable around town but it's better at a cruise. Not superb, mind...
Is there anything else that can cause this reaction other than a duff TMAP sensor or the wiring loom? I've cleaned the ICV, no effect (barely any crud in there either, heh).
Sugest you have a wet plug and socket on some part of the loom. It does not need to look wet to give grief! Have you been near a pressure washer lately? (last couple of weeks)???
Take ALL apart, dry for several hours each with fan heater or hair dryer, and I bet it goes away... Been there before.
Like you've realised, this is because the EMS is not getting a sensible reading from the MAP. Either the sensor or the wiring is wrong.
Well sensors are generally either faulty or dirty, and wiring can be intermittent. Beyond that it's unlikely that the EMS is faulty and you don't want that to be the case anyway.
One last thing you can try, force a retrain by disconnecting the battery. The EMS will be reset and will have to relearn all the compensatory thingamies. It'll run rough for a bit until it does. If it is software related, this will generally clear the problem, but I wouldn't expect it to help. Remember to clear the DTC after you've fixed things to see it if comes back, it'll keep being there until you do and people have been known to replace sensors a second time when the DTC continues to be reported because they've not actually cleared it.
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