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I have an S4 Spider. The idle drops and returns to 800 rpm, then drops down and returns. Eventually it stabilizes. It runs fine until the car comes to idle. What I have done. The vacuum hoses have been gone over. Some replaced and the rest are good. I replaced the o2 sensor since it was reading out of range and the engine light came on. I replaced the temp control valve only for good measure. I replaced the intake rubber that connects the MAF to throttle body. Most electrical connectors cleaned, deox-it and greased. New fuel filter and air filter. I cleaned the idle control valve. I can't get the symptom all the time. It comes and goes.

Any direction? Each time I replace a part the problem goes away for a while as though its been repaired

Please let me know
 

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An idle control valve can get a flat spot inside and wear out. You might want to replace with a known good one (used) to save money, but you can find non-oem ICV's for ~$120 on Rockauto.

I read an article years back in Roundel magazine, the BMW car club wrag, where the 'Hack Mechanic' chased down a problem like this. He wound up with a faulty ICV, which made his troubleshooting last longer.
 

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Comes and goes

You may be right and its a great thought. I started to think the pressure building in the gas tank may have something to do with this. A thread here discussed blockages in the one way valves near the gas tank. I do have quite a bit of pressure in the gas tank noticed when I remove the gas cap. Blowing into the valve was easy and did't appear blocked. I thought this since the idle is better with a full tank of gas. I will fuss with the valves again later. I have not checked the in tank fuel pump. The primary pump (at the rear axle) is loud but threads here imply they are all mostly loud. My problem is that with little experience with Alfa's, I have no baseline to understand what normal is. Back to the tank. I drove the car without the cap on the tank with a half tank of gas and problem persisted. Put cap on and same. Filled tank with gas and the problem lessened in frequency and impact. That is to say the idle variation was not as radical a drop off. These are observations and may have nothing to do with problem.
 

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Filled tank with gas and the problem lessened in frequency and impact. That is to say the idle variation was not as radical a drop off. These are observations and may have nothing to do with problem.
You might be on to something. There is a rubber connector on the intank fuel pump that "could" be going bad and there isn't enough pressure being developed as it is leaking back into the tank. I wonder when the in tank pump was replaced? The symptom of running better with a full tank is what we find on 164's with the rubber hose going bad in the tank. Not saying this is the reason, but a suggestion.
 

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Discussion Starter #5
Update...I cleaned the idle control valve again. It looks like new. I don't know how to test it. I went over all the vacuum lines. I replaced the in tank pickup, pump, and rubber step tube that has been known to leak. I verified the pump is working. The idle has improved but it still has a fluttering idle when coming to a stop. It isn't as dramatic. I drove a couple of days. Tonight I fixed the heater and ran out of ideas on the idle. I drove it for about an hour and it never fluttered after the heater repair. It was doing it earlier today though. I know the heater repair and the idle flutter are not related. I will drive it more and see if it has gone away or will be back. I'll post with updates. Thank you for your ideas. Keep them coming...I don't thing this job is done.
 

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What year is your S4?

How did you inspect the vacuum hoses? Visual only or pressure test?

What about just increasing the idle to 1000 and see if that helps?

Good luck,

Vin
 

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Discussion Starter #7
1991. I thought about increasing the idle. I decided to find what is going on. I'm weird that way. Its annoying but I enjoy learning about the system. I didn't want to just throw parts at it though. My technical expertise is weak regarding fuel injection and the electronics. Great opportunity to learn. I replaced the in-tank pump since the main fuel pump was very noisy. I could not hear the pump in the tank. I figured if I was to pull the in tank pump out to inspect, I might as well just put new parts in and eliminate that as an issue. The new in tank pump is audibly working. The main fuel pump has become quieter. The surging on the engine did decrease but still remains. I expected several problems since the car sat so long. I'm assuming there is a coil that operates within the idle control. Perhaps some windings are bad. I found a new bosch idle control valve on eBay and purchased. Once I get it and install I'll post an update.
 

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Oh. I did not pressure test the vacuum lines. I used ether initially. I removed each one and visually inspected. I cut the ends and coated each one with silicone grease and reinstalled.
 

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I read that thread before. Brilliant idea! I want to try this sometime. Another update. The idle control valve arrived yesterday. It seems to have solved the problem! Before I get too excited, I need to drive it a bit more. I cleaned the old Idle control valve and each time it ran better but always returned to the same symptom shortly. I'll run it over the weekend and update. Perhaps another item crossed off the list!
 

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Another update. The idle control valve arrived yesterday. It seems to have solved the problem!
well, that'd be good!

just for fun, check the resistance across the terminals of the old ICV. You should get something between 7-11 Ohms if the internal windings were ok.
 

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Discussion Starter #12
Car was driven and ran with proper idle all weekend. I will test the voltage but she is running proper for now. So happy. Drive-ability is great. I also want to test voltage from replaced valve. Thanks for all the help! I hope I can help others in the future.
 
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