Rough idle/surging - Temperature problem

Sat 06 Feb 10 13:51

Rough idle/surging - Temperature problem

Hi..
My C4 1990 is surging (revs increasing fora few seconds).
Looking under the bonnet, it appears to be because the air conditioning compressor is kicking in for about 1 second every 10 seconds or so,at which point the revs go up.
Not sure if its connected but the engine seems to take forever to warm up, the top hose gets warms soon after the engine is started and the main radiator fan comes on aswell..
Help

Sat 06 Feb 10 14:54

Re: Rough idle/surging - Temperature problem

I have no idea about C4's but some thoughts...

On other cars I have owned, when air con is switched on, main rad fan comes on also.

Could it be a faulty aircon switch or something? Is it possible to pull the fuse for the air con and then see what happens?

As for the engine taking an age to warm up, perhaps the stat has failed open. Any chance she got too hot last time you were out? Sometimes these stats are designed to failsafe.
As you are aware, the rad hose should be hot and not just warm.

Good luck! and hopefully a C4'er that knows what they are talking about will chime in soon!

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Sat 06 Feb 10 16:14

Had a look... Dunno if this helps but..
On one of the metal aircon pipes that run back toward the bulkhead (to condensor at a guess), there are 2 sensors side by side, a small one and a large one. If I pull the connection on the large one the compressor stops...
Small sensor 2 wires = Dark green with white stripe & black with white stripe
Large sensor 2 wires = light green & black

Thought I'd check out the state of my 02 sensor also, and the damn thing fell apart in my hands, what a day

Sun 07 Feb 10 01:57

Re: Rough idle/surging - Temperature problem

Rokin wrote (View Post):Looking under the bonnet, it appears to be because the air conditioning compressor is kicking in for about 1 second every 10 seconds or so,at which point the revs go up.
Not sure if its connected but the engine seems to take forever to warm up, the top hose gets warms soon after the engine is started and the main radiator fan comes on aswell..
The computer knows the A/c is on from the signal it sees from the connector you disconnected
When the A/c is on the computer will run the fans ,the computer also adjusts the idle when A/c is on so everything is working correctly.
Question is why is A/c on
Possibly a problem with the A/c head unit; do all the buttons work correctly ?.
In the short term , unplug the 2 wire connector on the compressor, that will prevent clutch from engaging

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12:1 all forged 383, LE ported TFS 195 heads, Vic Jnr 750DP, 234/236 cam ,410 RWHP, A4, 3.73

Sun 07 Feb 10 13:11

Cheers guys..

I took the thermostat out and tested it, and when it closes theres about a 1 mil gap all the way around the seal, so that would be the cause of the cold engine.

As for the aircon, the buttons on the head unit seem to work ok (as far as I can tell, I only bought the car a few weeks ago and I dont have a manual). I spoke to Keith at Eurovettes and he said that it may just be that the gas pressure is low, so I've disconnected the sensor as you suggested until I can get it topped up.

One other thing.. a previous owner has installed a switch that disconnects the heater blower motor, The blower works on all the different head unit fan settings. So why have a seperate switch. Any Ideas ?

Mon 08 Feb 10 00:21

Rokin wrote (View Post):One other thing.. a previous owner has installed a switch that disconnects the heater blower motor, The blower works on all the different head unit fan settings. So why have a seperate switch. Any Ideas ?

Should have said that up front
The blower control module (BCM ) controls both the blower fan and the compressor clutch .Obviously faulty on the fan side ( hense the cut off switch ,) possibly also faulty on the compressor side as well

Think of the BCM as a electronic ( solid state ) switch controlled by the head unit.
It supplies power direct from the battery to the fan and it grounds the compressor clutch circuit ( hot key on ) to make the clutch operate.
When faultly it will run the blower motor at random even with the key out of the car
giving a mystery flat battery in the morning

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12:1 all forged 383, LE ported TFS 195 heads, Vic Jnr 750DP, 234/236 cam ,410 RWHP, A4, 3.73

Mon 08 Feb 10 12:53

I'd seen posts about the blower going off at random, and wondered if it was that, but it doesnt seem to suffer with this problem.
Perhaps it used to be faulty and has been replaced.. Is there anyway to test it ?

Still on the subject of aircon, can I get my system topped up if its R12 or must I get it converted to R134 ?

Tue 09 Feb 10 00:10

Rokin wrote (View Post):wondered if it was that, but it doesnt seem to suffer with this problem.
You answered your own question.
A switch has been installed to isolate the blower motor until needed
Usually when the BCM goes bad it will still operate when commanded but will also operate randomly Try working in the garage on your own late at night and have the fan start up by itself

Rokin wrote (View Post):I
Perhaps it used to be faulty and has been replaced.. Is there anyway to test it ?

Bypass the switch

Last edited by vetteoz on Tue 09 Feb 10 00:12; edited 3 times in total. [13 %]
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12:1 all forged 383, LE ported TFS 195 heads, Vic Jnr 750DP, 234/236 cam ,410 RWHP, A4, 3.73