mx5cartalk.com

Was on a club run on Sunday and boost was dropping off shortly after the start of each leg of the tour, with practically no boost by the end of day.

I figured it was the dreaded "bog solenoid" getting gummed up.

Got home and checked the vacuum/boost lines and discovered the dealer fitted cruise control actuator was T'ed off of the sensor hose. The CC uses an accumulator, about a litre in volume, to deal with drops in vacuum and times of boost. It works well enough for the CC. But the downside is it means there's a litre of air being compressed and decompressed with the boost sensor sitting at the highest point of the circuit. Anyone that's pushed and pulled air will have noticed that condensation falls out of it and that once it condensates it has a tendency to stay that way. Wet.

Now water gets hot in an engine bay and it evaporates, and water vapour tends to rise... do you see where this is going... yep, it goes all the way up the hose and settles in the sensor, where the tube diameter closes up before opening out into the cavity of the sensor. So one last compression before being decompressed again... and the water sits in the sensor until the sensor is so full that surface tension eventually stops the air pressure changes making any measurable difference at the sensor. Bugger!

Pulled it apart, dried the sensor. Got a good 2mL or so out of it, I guess, it's very hard to say for sure because it only has one 2.0-2.5mm hole to come out of. Took a lot of shaking and a few curse words.

Plumbed everything back together, blanking off the CC for now, and boost is back. Oh yes it is! And it must have been a tad low for quite while, it's a far more rapid effect now.

I'll be re-plumbing the CC accumulator vacuum from elsewhere but I can only imaging how it could have been playing funny buggers with the boost reading to the ECU. Maybe this is why I've been measuring 8.5 PSI using the OBD2 port and FORScan; it's possible the elasticity of the accumulator was causing an overshoot of boost, by the boost sensor reading lagging the true manifold pressure. I only had time for a few short runs but 7.1 PSI was the peak seen.

The CC accumulator had been that way since 2004, as best I can tell.

Some experimenting is due next fine day.

I've never heard of that happening before. You're lucky you found the cause.
I'd have never worked that one out.

Oddly enough I've had lots of $ spent teaching me about instrumentation, and that's how I make a crust, so it was an intuitive enough find and fix. For me at least.

But still odd none the less; the water in the sensor was not what I was expecting.

Just another reason not to bother with CC, if you ask me

Personally, I don't see the point of CC in an MX-5, or any proper sports car, and I've done some long drives in mine. But, plenty of people really really want it, so to each their own I guess.

Like Roadrunner said, good solve!

Ditto...

As a loose rule: I dislike sixth gear; it means the road is too damn straight!

I wouldn't have fitted a CC myself but it was there from new. So was an alarm, with manky backup batteries and it was always going off, all the time. The alarm went to the parts bin, in heaven, on the first night of ownership. (The alarm probably cost nigh on a grand when fitted new but, other than making noise, it was simply snipped out of circuit without consequence. What a Ken joke!)

The CC stayed though, mostly because it's more hassle to remove. Was trouble enough to slightly relocate the actuator when the metal intake elbow and new BOV were installed. There's precious little space in the stock SE engine bay as it is. Losing the alarm permitted the CC to stay.

The CC is rarely used, as I prefer to find roads that make it redundant. But I'll admit it probably has saved me a few bob by relieving the gravitational effect on my right foot, which seems to increase disproportionately when the engagement factor of the chosen course decreases. Alarmingly, this gravity fluctuation must be the same that causes drivers heads to droop when traveling the same kind of roads. So watch out for dozy feckers!
(Especially those insulated darlings in their supersized Special User Vehicles... You know the ones... they've no regard for white lines and need to shed at least 30% of the speed limit when the road is no longer following a geodetic line, only to accelerate at least 10% above the limit once their steering wheel returns to its usual state of redundancy... Gotta love 'em.)

Driving my SE back across the Nullabor I would have killed for CC.

Glad you suss'ed it out.

Riding across the Nullabor I would have killed for CC too...all four times!

Driving it the first time should have been enough for one lifetime... Would have been for most people...

Turn left and continue twelve hundred and something kilometres said the GPS... I thought the Ken thing had developed a sense of humor, but it hadn't...

An update after some living with it...

Cruise Control is now much more stable; it used to run out of range sometimes on huge hills. In the past it needed some size 11 assistance. Not any more. Is rock solid stable, with barely a 2 kph variance over any highway or freeway terrain whilst cruising in sixth gear.

For anyone contemplating installing a vacuum operated CC: use the port on the rear of the intake manifold, just before the check-valve for the butterfly valve actuator. (VCTS or whatever it's called.) Don't tap anything off of the MAP/Boost sensor hoses.

BTW:
I believe the original installation was done by the dealer. So 110,000kms of very average...

AzzA68 wrote:Oddly enough I've had lots of $ spent teaching me about instrumentation, and that's how I make a crust, so it was an intuitive enough find and fix. For me at least.

But still odd none the less; the water in the sensor was not what I was expecting.

Given your expertise in the area; slight off topic question: I think my electronic Boost gauge sensor is either dying or dirty. Sometimes reads incorrectly ("zeros" at +3psi then works fine but always reads 3-4psi high at any boost or vacuum. I removed it and hit it with some compressed air and it seems ok for now.
Is it safe to spray a bit of Electrical contact cleaner on these sensors to clean them or is that a no no?

Roadrunner wrote:....

Is it safe to spray a bit of Electrical contact cleaner on these sensors to clean them or is that a no no?

Most of these type of sensors use a sealed diaphragm pushing/pulling on the element producing the measurement. So using anything that leaves a residue is undesirable if that residue is likely to hold onto any other debris that might happen through.

I'd rinse it with alcohol if anything; doesn't dissolve plastics and will pull any water away with itself, and then evaporates cleanly away. It also wicks well using paper towel. Other hydrocarbons and organic solvents can damage seals and plastics etc.

If using any compressed air, never overload the sensor.

Another consideration is if it's taking a single or differential measurement... i.e. It's got two sensors, one reading atmosphere the other the hose pressure. If you're seeing an offset, in the reading, it could be the external port is blocked. (These devices self correct for altitude and barometric change.)

An offset could also be caused by a mechanical preload or distortion of the diaphragm. Depending on the gauge/meter sophistication, it might be able to adjust for the offset. So RTFM. The manual should describe the method for a zero point calibration.

Also consider causes of noise:
make sure the negative terminals are connected as per the manual, at both ends of the circuit. It shouldn't be using a chassis earth under the bonnet is my guess, without seeing it.
Also, where is the measurement being made... Back end of the manifold is notoriously noisy, in most cases... Shortly after the TB is considered better.

HTH

Cheers,
AzzA