[DML] Re: Intermittent revving, Vacuum leaks, etc
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[DML] Re: Intermittent revving, Vacuum leaks, etc

I'm going to try and help.

First of all about the last remark: oil and fuel have different smells. So, if you smell fuel, it's not oil. It could be that covers of the fuel tank isn't quite sealing so take a look at the fuel pump cover and the fuel sender and see if there are traces of spilled fuel there.

Next: you have an automatic (I determine that from your below info). Automatics should be left in Drive even at stop signs while idling. That's what an automatic is for.

There can be various reasons why an engine and/or auto tranny is not behaving. One is the GC (governor computer, the electronics that arranges the up and down shifting while in Drive), the others are fluid level, vacuum leaks (e.g. at the vacuum modulator) or a wrong pressure at the vacuum modulator. Then the cable to the GC may not be correctly adjusted and so not follow kickdown precisely.

The idle control is composed of a number of parts (ISM, IS micro-switch, Thermistor and ECU). So each of these parts can influence idle speed behavior. Info on the control system can be found at D:05:01 and on.

Irregular revving usually has to do with the fule mixture not being correctly set. However be aware that other malfunctioning parts, like a vac leak or an improperly working WUR/CPR, can also contribute to that (I have not looked at your previous messages about the subject).

About the oil filler cap: on most cars it will not be an entirely tight fit. You could replace the appropriate O-ring for better fitting, but IMO this is not a reason for the behavior you describe.

Hopefully this is helpful to your investigation.


--- In dmcnews@xxxxxxxxxxxxxxx, "Michael Conrad" <mike@...> wrote:
> First off, thanks to everyone for links to the relevant docs when I posted  
> about this 2 months ago.  I'm back with (hopefully) better questions, and  
> a few new data points.  (and sorry for the late reply, but life has been  
> getting in the way of hobbies lately ;-)
> I've been paying a lot closer attention to what the car is doing lately.   
> I've noticed that when the car is "behaving", it actually drives pretty  
> well.  Most of the stuff I was talking about was related to when its  
> severely misbehaving.  When I mentioned electronically adjusting the idle  
> to match RPMs of the next gear, I actually was referring to normal  
> up-shifting; when the car is misbehaving, as soon as I let off the gas and  
> disengage the clutch, the engine RPM drops like a rock to about 300rpm in  
> about 1/10 of a second, and then starts bouncing.  Normal manual-trans  
> cars sort of "float" downward, making a nice smooth shift.
> Anyway, I did have one really interesting event that will hopefully help  
> diagnose this stuff: I was going about 40mph down a road, coming up to a  
> stoplight, the engine was fully warmed up (just above the middle white  
> tick mark on the temp gague) and so I shifted out of gear and coasted the  
> rest of the way.  My foot was completely off the gas.  The engine was  
> revving between about 500rpm to 1300rpm at one second intervals. (about as  
> bad as it ever gets)  At the exact moment I came to a stop (a pretty  
> gentle one, at that) the idle suddenly snapped to 750rpm and purred like  
> the problem had never existed.  It drove perfectly fine after that.
> Is there any part of the idle control that is based on actual vehicle  
> speed?  I'm having a hard time imagining that a gentle stop is shifting  
> something physical that starts or ends a vacuum leak. (with the Eibach  
> shocks I get much worse physical shakes just from the roads).  Maybe its  
> some kind of switch and just a random coincidence that it changed right as  
> I stopped?
> One other detail- I read that the entire engine is placed under vacuum so  
> that the oil vapors get collected into the vapor canister.  My engine oil  
> cover isn't very tight (I can wobble it around with my fingers while its  
> fully seated).  Could that be a potential problem?
> Also, I get a fuel smell in my garage overnight, but I've always assumed  
> it was just the oil that leaks out the rear seal. (thanks to Prev Owner  
> using super-thin synthetic in a 100Kmi car)  Is there some way I can test  
> the vapor recovery system for leaks?  Could that even be the problem?
> Mike Conrad
> vin 5732


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