P38A Air intake temp readings...

Hello everybody,

I hope lock-down isn't sending you all mad. Being locked-down means I've got some time on my hands and so I'm going to sort out a few niggles with the beast.

I have some rough running problems. Feels like a misfire, but only happens when well warmed up, and under some load. I've had the Nanocom on it and there are no faults logged anywhere. The only anomaly I can find is my air intake temperature is reading -10 when the engine is cold, and it only got up to +5 when thoroughly warmed up. Is this reading in degrees centigrade? It certainly isn't that cold outside, but I thought it might be "steps" or something other than degrees C.

The only other thing I can find is a slight chug from around the left hand CAT when fully warm, so I think I've got a the start of a blowing exhaust from there. Can all this cause rough running?

MrGorsky said:

Hello everybody,

I hope lock-down isn't sending you all mad. Being locked-down means I've got some time on my hands and so I'm going to sort out a few niggles with the beast.

I have some rough running problems. Feels like a misfire, but only happens when well warmed up, and under some load. I've had the Nanocom on it and there are no faults logged anywhere. The only anomaly I can find is my air intake temperature is reading -10 when the engine is cold, and it only got up to +5 when thoroughly warmed up. Is this reading in degrees centigrade? It certainly isn't that cold outside, but I thought it might be "steps" or something other than degrees C.

The only other thing I can find is a slight chug from around the CAT when fully warm, so I think I've got a the start of a blowing exhaust from there. Can all this cause rough running?

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Petrol or diesel and what year? The temperature reading does not sound right.

V8 Gems Petrol. MY1998.

MrGorsky said:

V8 Gems Petrol. MY1998.

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Not familiar with the V8 but if it has a MAF, the inlet temperature readings probably come from it.

Sensor is located on the side of air filter box.

Reading the Nanocom instructions suggests it's in degrees. Not very clear though.

So my sensor is probably duff then. What difference would it make to the fuelling do you think??

MrGorsky said:

Sensor is located on the side of air filter box.

Reading the Nanocom instructions suggests it's in degrees. Not very clear though.

So my sensor is probably duff then. What difference would it make to the fuelling do you think??

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I don't know the V8, but I would expect it to affect the mixture.
The inlet air temperature sensor on the diesel is in degrees C on diagnostics, so I would expect yours to be the same.
@wammers would know for sure

IAT reading off is a common problem on the GEMS. Maybe what's happened is that the resistance of the wiring has increased with time, or perhaps Nano is playing tricks with us. The 98 GEMS IAT is separate from the MAF, and supposedly has little influence on the ECM.

It has a default value if it fails and will retard the ignition above 55 degrees. On the basis I figure that the ECM might use it to calculate the density of the air in combo with the MAF I put different resistors across the IAT plug to see what Nano/ ECM interpreted them as in terms of temp. I also took the existing sensor and a new sensor and looked at the resistance to temp curve of both of those.

Having the IAT so far off whack may have some unintended consequences (air density, etc). Since it's not possible to reduce the resistance of the set-up, I made a short section of twin wires with a male and female JPT connector at the end, and soldered a 5.1K resistor across the two wires ie in parallel - bit of a fudge from an electronics point of view, I know, but i wanted to keep it simple. The section can be removed easily, but gives me a temp jump of about 13 degrees. 20 degrees temp difference is about 10% in air density. Not sure that GEMS uses the IAT data for air density, but if it does, then I'm on it.

If its MAF next time it does it, pull over and unplug it - should run on default see if it cures it

RangeRoller dt said:

If its MAF next time it does it, pull over and unplug it - should run on default see if it cures it

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Unplugging the MAF did nothing. He tried that before I lent him the Nanocom.

I reckon the exhaust leak isn't helping ...

EXhaust leak will throw oxygen sensors out.

Freezespray on cam/crank sensors check for culprit perhaps?

IAT shouldn't make it misfire, especially only when hot.

My Money would be on the CPS, being an aged Gems unit, 'tis about due..

Check the Lambda reading too, healthy sensors will fluctuate between 0-5v if it is a fixed measurement then they're duff.

Yes, CPS has also been suggested!

I prefer to fix the known issues first though.

Exhaust leak definitely will not help. Attached doc explains lots about GEMS ECU for a V8 install in a Morgan. Some differences, but useful info on how GEMS works.

Agreed -the David Poole doc is v. useful

Mine 1995, 4.6, 198k, does the exact same thing and I have started to consider a blocked fuel filter.

oakey said:

Mine 1995, 4.6, 198k, does the exact same thing and I have started to consider a blocked fuel filter.

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Is it heat dependant?

Dont think so it just starts to "misfire" when pulling hard . Never tried with a cold engine

Oakey - if you are idling, bring the revs up, and then take your foot off quickly do you get a miss during the deceleration. Or the engine needs to be under load? Mine used to do the former, but not the latter

oakey said:

Mine 1995, 4.6, 198k, does the exact same thing and I have started to consider a blocked fuel filter.

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His has LPG so I assume the issue is on both. Plus, Gorsky's is serviced very thoroughly and regularly so I doubt the filters are very old.

I'm still on cam sensor but best sort the exhaust leak first.

Thanks for all the replies. Lots to sort out and consider. Is there a way to check for Cam Position Sensor operation? Or do I just change it and hope it fixes it?

The exhaust is certainly far from perfect. I can't feel any obvious blowing, but it's chugging slightly, and rattles a lot when you take your foot off the gas. I can't check for long because it gets too hot to have my hands under there after a short while. Perhaps it's all full of small perforations. It's been on there 22 years after all!