Help!! Bad misfire from cold for 20-30 mins

[oxfordpaul 0]

Car starts fine and idles fine but mis-fires when touch the throttle and will not rev more than 2-3000 rpm when really cold, either stationary or moving. The longer it has stood the worse it is and the longer it takes to clear. When driving I have to change up gears really quick as it simply will not rev more than 2-3k rpm. Once really warmed up it just has the occasional miss and then is fine. Checked plugs and they are fine. It's a 95 MY VR6 and suggestions have been leads or coil pack - both are expensive so how do I know for sure?

[theRuler 0]

my car did that just before the head gasket went. although not quite as bad as you describe.

its a tricky one to call.

 

new leads and plugs are always a good investment. and while you have the plugs out, they can sometimes give you an indication if a head gasket is leaking.

 

my car also ran like that another time.after changing leads and plugs (to no avail) i took the airflow meter off, and tested it on another car.

when i refitted the airflow meter, it was all cured!! almost like it need unplugging and plugging back in.

 

its now running like a dog again until it is warm, but think i have a fuel leak that aint helping (robbing the fuel pressure overnight)

 

my car has done 150k.

[Ice White Socks 0]

Sounds like a sticky ISV to me. I only ever had this problem when nailing it from stone cold (I know, I know- I don't do it often :oops: ) at which time it would rev to 4k and then suddenly die in power and run badly until the engine warmed up.

 

Whip the ISV off and give it a clean out with carb cleaner followed by a squirt of WD40 to finish. I would definitely do this before investing in either plugs or a coil pack.

[theRuler 0]

forgive my dumbness, but what is the ISV ? :oops:

[Ice White Socks 0]

No worries- Its the idle stabilisation valve. It sits under one of the top plastic engine covers and I believe it regulates the idle by controlling the air flow somehow. It has a valve which opens and closes using an electric motor/ solonoid. They get coked up with crap and start to stick causing idle/ power problems.

 

To get to it you take off the top bank of plug leads (either use a tool to remove them at the plug end, or pull them out of the coilpack end), Use a spline key (tho an allen key will do apparently) to remove the plastic cover bolts. You'll see the ISV under the cover with a couple of pipes going into it- undo the pipes- clean out with a rag and carb cleaner- squirt some WD40 to finish and reassemble. Someone might have a picture of exactly what it looks like if I haven't described it very well

[vr6storm 0]

Car starts fine and idles fine but mis-fires when touch the throttle and will not rev more than 2-3000 rpm when really cold, either stationary or moving. The longer it has stood the worse it is and the longer it takes to clear. When driving I have to change up gears really quick as it simply will not rev more than 2-3k rpm. Once really warmed up it just has the occasional miss and then is fine. Checked plugs and they are fine. It's a 95 MY VR6 and suggestions have been leads or coil pack - both are expensive so how do I know for sure?

 

well tbh its not exactly being mechanically sympathetic reving the VR over 3k when really cold........best wait til the oil temp is over 74 degrees before going that high IMHO

[theRuler 0]

To get to it you take off the top bank of plug leads (either use a tool to remove them at the plug end, or pull them out of the coilpack end), Use a spline key (tho an allen key will do apparently) to remove the plastic cover bolts. You'll see the ISV under the cover with a couple of pipes going into it- undo the pipes- clean out with a rag and carb cleaner- squirt some WD40 to finish and reassemble. Someone might have a picture of exactly what it looks like if I haven't described it very well

 

good looking out bro i'll try that 8)

[oxfordpaul 0]

Well irrespective of how hard you should rev it when cold, it shouldn't run like a bag of s**t!! I've never changed up below 2000rpm now matter how cold the engine and never had any problems. I'll try the idle valve thing first then (thanks ice white) and let you know how I get on...the only doubt I have is that it idles perfectly and on previous cars the ISV has led to stalling when cold.

[dr_mat 0]

Erm, if it starts and idles fine chances are the ISV is perfectly ok. Sounds more like a temp sensor problem to me.

[maff 10]

I have exactly the same problem, car starts fine then after about 10 secs it starts to misfire slighty and lasts for about 5 mins then its perfectly fine again. I have been told to check the cold temp sensor but would this cause misfiring?

[dr_mat 0]

Over or under fuelling would. If the engine thinks it's a warm start and it isn't it's going to be running a bit lean!

[oxfordpaul 0]

My local garage think it's electrical - coil pack is favourite but have got to put on VAG.com to see what that brings up. I don't think it's fuel cos it's perfect at idle and up to about 2000rpm.

[emax 0]

It's a 95 MY VR6 and suggestions have been leads or coil pack - both are expensive so how do I know for sure?

 

You could maybe try swapping coil packs/leads with someone in your area with a VR in order to diagnose the problem? Another one I've heard of is checking the coil pack in the dark with the engine running. Apparently, you can see the occasional electrical spark on a failing unit.

 

When my coil pack went, the car 'suddenly' started to run on 4 cylinders. I drove it the remaining quarter of a mile to work and parked it up. When I went out to start it a few hours later, it fired on all six. I think when a failing coil pack cools down, it will work again for a short while until some heat gets into it. At least that was my experience.

 

Interesting that you're getting these symptoms from stone cold and it gets better when the car warms up.

[theRuler 0]

Interesting that you're getting these symptoms from stone cold and it gets better when the car warms up.

 

if the head gasket had a leak, it could gather moisture in the cylinder while cold, then it all tightens up and seals when warm?

 

or if it were a fuel pressure / fuel leak issue, once the car has been run for a while the pressure builds back up?

 

maybe.

[Ice White Socks 0]

Erm, if it starts and idles fine chances are the ISV is perfectly ok. Sounds more like a temp sensor problem to me.

 

I understand why you say that but I had stalling issues AND cold running issues. Cleaning the ISV solved them both. You could feel when the ISV had stuck out of place by the rough running and and lack of power at the top end. When the engine warmed up it was fine. Just my own experience

 

If I had any running issues with the car- the first thing I would do is spend 10 minutes cleaning the iSV, before spending the thick end of £200 on coil pack and leads.

 

Yep agree with you on possible temp sender though

[Kevin Bacon 5]

Bang on Ice-White socks. Most people wrongly assume it only affects the idle and nothing else, so overlook it when fault-findling other running problems.

 

And everybody I've seen that's changed the Blue temp sensor has had zero affect. You will know when the blue temp sensor is out.

[Ice White Socks 0]

Bang on Ice-White socks

Cheers 8)

[dr_mat 0]

ISV shouldn't effect throttle-open situations, but it'll still effect those times when changing from throttle-open to throttle-closed and vice versa.

It seems the cold temps have been killing off a few VR6s just recently...

[Kevin Bacon 5]

Yes it does because if it jams open (as Ice white's did), it creates an air-leak past the throttle.... It happens a lot. Check the carbon scoring on the internal vane, it's just a matter of time.

 

My VR was wicked last winter, not affected by the cold at all. Thoroughly enjoyed throwing it about in the snow and ice :lol:

[dr_mat 0]

True, if it sticks open...

I'm not sure which way it sits when switched off - fully open or fully closed?

[Kevin Bacon 5]

When the ISV is off, it's fully closed. It should only ever be open when idling......(I'm sure Blue Nothelle will be along to correct me shortly!) but due to dirt ingress, it sometimes sticks open causing rough running or sticks closed, which stops the car from idling. And they absorb a tonne of heat from the exhaust manifold, which doesn't help matters.

 

I would prefer to use an Auxillary air valve from an 80s Audi engine but finding one big enough for the VR is tricky.