1985 BMW 318i with eratic idle issue


So, the idle goes up, then down till the car almost dies with about a 1 second period. Also, the fuel economy gauge bounces in time with the idle. Whats the deal?

Open up the hood when its doing that…hear any hissing noises? Could be a vacuum leak.

That reads like a fragment out of the middle of a much longer story. The rest of the story would have had things about mileage on the car, anything notable about its history (e.g. engine replacements/rebuilds if any, any recent major work), the history of this problem (e.g. how long its been doing it), how the car behaves off idle, notes about the basic state of maintenance…stuff like that.

In any case, I’d say that you just have a good old-fashioned vacuum leak.

If you do hear a hissing noise of some kind and cant pin point where it is coming from…I usually use a can of brake cleaner…just spray it around all the different vacuum hoses…You will be able to tell where the leak is that way cause the car will really die down when it starts sucking in brake cleaner. Works well for stuff like that.

Check the air intake hose between the air vane meter and the throttle body for leaks/cracks. If that checks out okay, then there may be a problem with the air vane meter itself.


Thanks for the help guys. I’ll look into everything mentioned above, especially vacum leaks. I just thought that since it had this weird periodicity to it that might change things. Found some creative ways to find vacum leaks late last night so I’ll try them out. Sorry for the short story cigroller, I actually don’t have much more information about the car than that. It is all stock and I just got it. I picked it up for a song, so I knew about the idle issue at purchase. Mileage is between 160 and 200 k, and it has the M10 engine. Odometer is broken and it’s sat for awhile. Just digging in and this is the first major issue I want to clear up and I hadn’t seen anything quite exactly like it before.

Again, sorry not to include more information. It was late when I posted and the site wasn’t cooperating with me and it rejected the post a couple of times before it actually went through so I was a little impatient with writing the post over and over again at that point. Again, I’ll check heavily for vacum leaks and I’ll post back.

Actually, by the way…

Since this is a new car to me, quite used to anyone else, I am planning on doing a big tune up, lots of catch up maintenence like I do with every used car I get. At the moment I’m planning plugs, wires, cap and rotor, fuel and oil filters, oil change to full synthetic european “blend,” air filter, and…

what else? Something I’m missing that might be particular to a BMW that anyone would recommend? I’m pretty good with this stuff but I’m no master mechanic, I managed a clutch on a Honda Civic and a timing belt on an Audi TT though, just new to BMW and it’s been awhile since I’ve worked on a car this old.

Thanks again in advance for any advice anyone can offer.

TESTER has a fantastic suggestion…That hose going between the MAF type device…or air vane for the injectors…you will be all out of whack…and that hose is VERY OFTEN found with cracks in it… Those BMW’s cannot tolerate a leak there at all. I’ve amassed many Duct tape cores from over the years trying to patch those hoses on other guys Bimmers…just to get home…thats for sure. Duct tape will get you by for a while till you buy another hose…LOL I can attest to that…


Inspect the brake lines. They may need to be replaced. And whether or not they need replacement, change the brake fluid. Also, check the coolant.

Good suggestion on the brakes. They did feel a little mushy. Forgot to mention the coolant, but that was definitely on plan.

How about transmission oil? I always hear mixed opinions about this. Do or do not do? I’m thinking do… (It’s manual)

As for the trans…AT LEAST check it. Pull the TOP bolt on the trans and put your finger in there to see where the level is…IF oil starts coming out QUICKLY put the bolt back in and maybe leave it be. I say this because the BMW oil from the dealer is pricey…and SHOULD be the only stuff you use in there. So unless you have 3Qts of it handy, please don’t replace whats in there with an inferior oil please…those trannys last a long time when given what they ask for and since they very rarely ask for anything…at least grant them their one wish.


@Honda Blackbird: Good to know on the trans, I prolly woulda researched it anyway, but I did not know that and it’s good to know that I may have to order some special oil. Transmission may have to wait. I think I’ll just go ahead and replace it. I figure most people never do, so I’ll make sure it gets done. Really want that long life.

Double check me on the fluid…but I recall one where i needed Whatev BMW told you to use…I could be wrong, but do the research and find out…If I’m wrong…no prob…carry on… Just wanted you aware that it IS a possibility for sure.

and youre right about people not ever changing it…It DOES last a very long time, but if you are of the mind to think of it…then by all means change it…you will feel better, so will the trans…they will last and last those manual ZF trannys…are nice pieces, very well made. I believe its a ZF in there…BMW been using those a LONG TIME


@Blackbird: Will do. I’ll have a manual and since I’m no expert mechanic I tripple check everything I do anyway. I almost consider it a challenge to see how well I can get a car of this age running well and to see how long I can keep it that way. If this one proves to be half the trooper I expect it to be it will either end up donated to a young cousin fresh driver or maybe I’ll keep it for myself. I did see a write up on a guy who added a turbo. Interesting prospect…

Anyway, back to the original issue for a moment. I’ve been looking into the vacum leak issue, but looking at the YouTube videos it doesn’t seem that the vac leaks tend to cause the pulses I’m seeing in the fuel economy gauge. I’ll be checking for those, but it seems like that should be another clue. I’m not sure what the pulsing of the fuel economy gauge can tell me, but whatever it can tell me I’d love to hear it.

Check the throttle position switch. It is, I believe, just an idle indicator switch, i.e., when the accelerator is released and the throttle drops fully back to idle the switch should ground the circuit to signal the ECM to control the idle speed. But please, I am working from memory. My SWAG is correct if there is a single wire at the throttle position switch. And if grounding that wire while the engine is idling suddenly improves the situation the switch is the problem.

Fantastic Red Knox! Finding that sucker just moved to the top of my priorities as well. Sounds like a logical winner for my issue.

Your issue actually behaves exactly like a standard vacuum leak. That’s what happens - the idle surges & falls. A large enough leak will stall the car. I’m not saying it might not be other things. I’m just saying that what your car is doing is exactly what it will do with a vacuum leak

@cigroller: But it does seem like the throttle may be involved since it never dies and the fuel economy gauge dances around in time with the waves of surging idle. I’m certain I’ll be checking both, and I’m glad I got Rod Knox’s suggestion. This way, whoever is right I find the problem and that’s what I really want.

Yeah Rod is correct about that position sensor…its an adjustable micro switch on the TB…

That fuel miser gauge is actually a simple Vacuum gauge…go online and look up “how to interpret a vacuum gauge” in relation to a running engine… This may indicate a hanging valve or some valve adjustment issue…and of course another Vac leak.

That’s me guess…the intake tube will affect how it runs NOT your vacuum gauge/fuel miser thingy…I know what gauge you are talking about.


That reminds me, I read that these engines require a valve adjustment every 30k, which is also on my list of items to freshen up the car. Sigh… better set aside two weekends.