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Hi Vacuum gremlin

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I am running a 7liter Ford FE with Borla ITB system, fuel injected. It is running in Alpha-N mode with MAP compensation. At idle, it pulls around 60KPa, at cruise, I typically will see around 80KPa.

I am chasing a gremlin that occasionally something in the engine goes haywire and it starts pulling 35KPa - or at least that is what the ECU log is reporting. I don't know if it is a defective MAP sensor or something causing that reading. It occurs when running on a flat surface, with no changes in load or TPS. The logs are showing a vacuum increase over about 1-1/2 seconds. With the MAP compensation enabled, it responds by pulling about 40% fuel out which causes the engine to stumble/choke

Does anyone have any ideas on where to begin to look for the culprit?



When trying to get a clean MAP signal from ITBs, I think it's important to have all runners the same distance from the map sensor. Not sure what your plumbing looks like, or whether you have a pressure reservoir or are using small orifices for each cylinder (these are other ways to smooth/average the MAP value).

I would look for features in your ECU that allow either averaging / filtering the MAP signal, or sampling the map sensor at a specific crank angle.


The manifold for the ITB's has a common plenum in the valley area and each runner has about a 3/16" (5mm) hole from the runner to the plenum. The port is then at the rear of this plenum with a vacuum hose to the MAP sensor. The size of the plenum is approx. the underside of the manifold (8 x12 approx) and about 1" deep.

This is a picture of the bottom of the manifold, the red line is the port to the sensor. The cover plate is removed to view the various ports.

Manifold bottom - vacuum plenum

What has me confused is what can add 35KPa of vacuum with no changes being made to the motor, induction or load and that additional amount being 25KPa below what the engine normally idles at.

David, the manifold seems to use some sort of b******isation - "Cast in A356 alloy and machined in precision CNC equipment the manifold has cast-in plenum facilities for the MAP sensor and the Idle Air Control" - I can't find, with a preliminary search, any details on this.

my thoughts were if there may be some vacuum operated device occasionally tripping a threshold, such as a brake servo'/booster valve?

The only other item on the vacuum side of thing is a PCV valve. No other vacuum devices.

well, that's a huge vacuum chamber! I would suspect the sensor (can you try another?), or wiring (intermitent power/ground/signal might result in a signal drop to some default value).

I have forgotten are your ITB's controled by DBW, or is the a cable/manual linkage? If DBW is could there be fault closing the throttle for an instant?

Is coolant temp or TPS changing as well? Is the sensor internal to ecu or remote mounted?

It sounds like an earth loop/resistance/shared current path issue. Earth reference on MAP is lifting or sensor power being pulled down. Can you confirm if anything like fans or A/C clutch or something is switching during this event?

I've tried checking all wiring and can't find a short or intermittent. I'll try a different sensor, not sure if it could be at fault or not since it appears to be working fine except this drift.

The ITB's are controlled by a manual linkage.

Coolant and TPS reading are constant during this fault. The sensor is remote mounted, a GM 1Bar unit connected to the manifold via vacuum hose. No other electrical items coming into play, the only one possible is the coolant fan and it is off. I've traced all the wiring but can't find a problem, the sensor is grounded to the ecu thru the sensor ground.

Maybe dud sensor then, sounds like you have covered a lot of checks.

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