E28 Starting RPM too high

[Alex Jenkins]

Hello there,
I have a little problem with my '87 528e and I was wondering if someone may have the fix.
It rev's up too high on cold starts.
It consistently does not start on the first 5 sec. crank attempt and then winds up to approx 1700 rpm when it starts immediately on the second try. It wont do it again unless it's off for 3-5 hr.'s but that depends on the weather it seams. On a hot day, it will go longer without starting harshly.
It also idles too low, around 550rpm in gear, until it warms up for 5 minutes or so but I don't know if that's related.
Hot restarts peak at about 800 rpm.
I've suspected the fuel system and have tried replacing both pressure regulator and idle solenoid to no avail.
Could it be a vacuum leak or a sensor?
Thanks for any good advice!

[Tidybuoy]

I would reccomend checking all of your vaccum hoses. If they haven't been replaced recently (past few years) they are probably due. When ever there is a vaccum leak, the engine is receiving "un measured" air and the computer is not adding the appropriate amount of fuel. Therefore, the engine will run lean (usually causes high rpms).
Besides vaccum hoses, other sources of leaks are the fuel injector seals, oil cap & dip stick.

You mention that it only does this the first time or after the car has sat for 3-5 hrs. Could be a sensor that is malfunctioning (temp sensor or thermo time sensor).

Also, your car has an idle control module which looks like a small coil & is mounted near the air intake system. it could be sticking and can be removed & cleaned with carb cleaner.

Oxygen sensor - these are only good for about 50-60K miles. The oxygen sensor measures the exhaust and adjusts the fuel injectors based on what it "thinks" the engine is doing.

Do you know if you have Motronic 1.0 or 1.1? the later 1987 & 1988 cars are Motronic 1.1. If you don't know, you can tell by looking at the diagnostic input. The 1.1 setup has a large diagnostic terminal about 3" in diameter with a large screw cap. The 1.1 system does not use a cold start valve - the 1.0 does. Instead, the 1.1 has different injectors that pulse differently during cold startup and the various sensors are critical to proper engine management.

[Jared at Pelican Parts]

mostly likely either a vacuum leak or a sticking idle control valve.

[Alex Jenkins]

Hi,

Thanks for the feedback, I'm going to take another good look this weekend.
As far as the idle control valve goes, replacing it didn't help the problem or my pocketbook.
Quite sure it has a cold start valve right there on the left side of the intake next to the idle valve (braided fuel line, not an injector, banjo nut). Wouldn't the O2 sensors be uninvolved on the initial cold start, until some sensor tells the ECU it's time?
I really suspect a vacuum leak and/or some sort of temp sensor. The other day I got home and caught it hunting a little bit, maybe 100 rpm.
I've listened all around before but found no hissing. Removed that little line that draws on the climate controls, no change in idle. I didn't see any other likely hoses but I'll look real close. The injector seals sound very possible. They would be the originals as I have all records from previous original owners and haven't seen any job that would cover the injector seals. It has 132K after 17 yr.'s.
I can't see checking for vacuun leaks with carb cleaner on this car like old days with the plymouth. Should I try adding a few drops of oil to the seals before a cold start and see if that helps? I could spray can silicone at them but I think better yet I dribble some oil down a long screwdriver and dab a little in there.
Would a vacuum leak act so much differently, always on a really cold start and never on a restart?
So if I have Motronic 1.0, I wonder what to call and where to find and how to test this thermo device? I have multimeter. I've seen a little brass sensor, with crusty epoxy, around the thermostat but I imagine that it is the sender for the gauge.
Thanks again, I'll look some more and let you know.

[Alex Jenkins]

Oh yeah, my initial suspicion was related to fuel pressure. A local bmw mechanic friend said something about a fuel accumulator or something like that. Could it be possible that there is some fuel system component that is bleeding back into the tank. It never starts cold on the first attempt - almost as though it needs to prime and then get's a little extra unregulated fuel when it does start. Once again, the fuel pressure regulator replacment didn't help!?

[Tidybuoy]

There are several places that a vaccum leak can exist:

Tube from valve cover to air intake
Intake bellows
hose from charcoal cannister to purge valve
little hose from intake plenum to fuel pressure regulator
oil cap
dip stick
injector seals (remember to buy 13 - two per injector & 1 for the fuel pressure regulator - there all tha same & cheap)

If you definately have a cold start valve (you can tell because it would be coming out of the pleneum - just over the valve cover) then, you have Motronic 1.0 - not 1.1

Another sensor that can go bad is the throttle position switch. This switch tells the computer where the throttle is and when it goes bad it can cause bad idle, surging, or missing at about 3000 rpm (one symptom or all three)

I would start with the intake bellows. Take it off and inspect thoroughly. If it's ok, then just clean it up and tighten good.

Next, I would check the hose that goes from the valve cover to the intake. This one can look perfectly fine but you may find it's very brittle. This only costs about $15 to replace.

Also, make sure the bolts to the valve cover are tight. You will notice that if you remove the oil cap - well the engine is running, the engine will shut down - That's how critical vaccum leaks are with this engine.

You can probably test the cold start injector by crimping the fuel line to it & unplugging the electrical connector. It's possible that this valve is leaking; causing unnecessary fuel to be supplied.

Hope this helps & good luck.

Vern

[Tidybuoy]

In response to your last question:

Good point. There's no fuel accumulator on this car. There is on other fuel injection systems but because this is a "pulsed" system - it's not needed (but that's another topic).

However, since you say it's hard to start - maybe your fuel filter is clogged. Sometimes it takes a while for the fuel pump(s) to build up enough pressure to make it through the filter.

This is easy to change. It's located under the car near the fuel filler neck. (you may already know this but .....just in case you don't).

Take a look at the filter. And, while your there, inspect the fuel lines & smell for gas. German cars are notorieous for bad fuel lines. I don't think they have the same type of rubber over there. If the fuel lines have the original manufacturer type (crimp on) hose clamps - they definately need changing.

Vern

[Alex Jenkins]

Thanks, this gives me a lot to work with.
There is no fuel starvation on the top end (I love the way it breaks 100mph just under 3krpm) but I guess the startup pressure could be another story. I'll print this thread and get under the hood/filter this weekend.
thanks a lot, I'll preserve the lower end yet.
Alex

[Alex Jenkins]

Hello, please excuse my creative recollection. The cold start injector has a rubber hose and is secured with 4 allen bolts. The braided line and banjo fitting were a combination of other memories. I'll be sure and look at the engine before I say something like that again. Sorry and thanks.
I have't found enough to report on yet but I'll keep looking