Dropping RPM whilst standing in traffic

[Karl_T]

OK all, here's the problem. On a hot day if I get stuck in traffic and the car's stood idling, the RPM slowly drops and drops. Each time you move forward a few feet in the queue and then put the clutch down and come off the gas the revs will drop initially to 800, then 600, 500 as time goes on and they struggle to rise back up. Eventually the car just won't idle and it cuts out. Normally the rear side fan cuts in but not the front ones and the engine temp gauge isn't going up, but it seems like a heat related problem. Once you get going and drive normally for 2 or 3 mins all is well again.

What's causing this? And is there an easy fix?

Ta Karl

[LimeyMk1]

Seems to be a common problem Karl, I've had it occur on both my Mk1s when the weather's got hot. I have a feeling it is related to the cone air filters in the engine bay, as the heat in the bay increases the charge density decreases to a level which causes the engine to stall. Just a theory, I dont know if Mk1s with the original plumbing suffer from it.

Quick fix is to simply up the idle via bleed valve when the weather hots up!

Chris

[mr2mk1chick]

sounds exactly like my prob. have done a temporary fix by turning up the idle screw when the car was hot and with revvs falling. but i think my prob is having missing air sensors since K&N filter fitted without addign the sensors back in. will let u know on that one. check your sensors as this will give the ecu wrong info therefore the car could over fuel to compensate hence flooding and low revvs.....maybe

another reason for a missing / dropping idle thingy is having air in your coolant. need to flush out.

either way i'm not v mechanical, but hopefully will give you a couple of ideas.

[Lauren]

sounds exactly like my prob. have done a temporary fix by turning up the idle screw when the car was hot and with revvs falling. but i think my prob is having missing air sensors since K&N filter fitted without addign the sensors back in. will let u know on that one. check your sensors as this will give the ecu wrong info therefore the car could over fuel to compensate hence flooding and low revvs.....maybe

another reason for a missing / dropping idle thingy is having air in your coolant. need to flush out.

either way i'm not v mechanical, but hopefully will give you a couple of ideas.

Its only the idle up valve that gets removed when you put a cone filter on the throttle body.

Tbh it could be that the idle is simply set too low. You often find even on a 20minute journey that the engine isn't fully warm. Try adjusting the idle speed when the engine is nice and hot, this may well sort it.

The idle up increases idle speed when you have lots of leccy items on, ie lights, wipers, fans etc.. I run without the idle up valve with no probs. Yes the idle does drop 50-100rpm on high electrical load, but really its no issue.

[Karl_T]

I haven't fitted a cone filter in the engine bay <Shock horror> I went for a RAMair in the original box in the boot so the air's still coming from outside. I'll see if I can find an air temp sensor though and check it. You never know.

I changed the coolant just before the weather got hot so I don't know for sure which is the cause, but if there is air in there I'm bu&&ered if I can find it. I went through the phase with the wacky up down revs whilst I was bleeding it all out, but by the end of the morning I was pretty confident it was all out.

[LimeyMk1]

That's one of these yeah?



I still reckon you're going to have issues with too much hot air.

[JMR_AW11]

On my mk1b the air temp sensor is located half way along the plastic intake tubing. i.e. about a foot from the throttle body. The sensor is encased in a small plastic body, the tip of which sits in the airflow.

I have datalogged the readings from this sensor on a laptop and the sensor is quite slow to react. It is also prone to a degree of heatsoak from the engine bay if you get stuck in traffic queues for a long time.

This falsely heats up the sensor and fools the ECU into thinking the air is hotter than it really is. It will therefore lean off the fuelling more than it really should. This will affect the idle quality.

BTW I'm not saying this is the definite reason for your poor idle symptoms, but it won't be helping.
You could try tweaking your mixture screw clockwise a tiny fraction to see if this improves things next time.

[Karl_T]

That's one of these yeah?

Image Replaced With URL For Quote http://www.fensport.co.uk/partsfiles/aw ... rmr2-1.jpg

I still reckon you're going to have issues with too much hot air.

Nope, not one of them. I think mine's called a 'panel filter'. It goes in the normal box on the LHS of the boot. Seems to free up the revs at the top end nicely and doesn't need cleaning as ofter as a cone.

Karl

[Karl_T]

FAO JMR_AW11

Ta, sounds like that could be worth a go. I wonder if you can shield the sensor any, I might try that too.

Karl

[System-G]

My MKIa used to have a similar problem where after a long run and in warm weather the engine idle would drop until it just cut out. After adjusting the TPS and idle, it doesn't do this anymore. But since adjusting things it sometimes has difficulty reaching idle imediately after starting after a long motorway run (have to coax it a bit with the throttle and watch for it to stabalize at around 800rpm).

[feral4mr2]

I have a feeling it is related to the cone air filters in the engine bay, as the heat in the bay increases the charge density decreases to a level which causes the engine to stall. Just a theory, I dont know if Mk1s with the original plumbing suffer from it.

never had a problem with idle running a pod filter in my engine bay (which is the best place for it IMO).

[*DeathStar*]

my car is known to do this after a long drive. it has not of yet... cut out . but the revs do get very low...

ive never heard the front fans kick in... at what point should they be working?