Sunday 6 June 2021

E30 318i [M40]: Massive air leak SOLVED - lower inlet-manifold gasket

I had checked and re-checked the idle control valve [ICV], the air-flow meter [AFM], ignition system, tested for the fuel pressure, performed tests on the DME [ECU] to make sure that was working and even laboriously tested the wiring loom for continuity, but still the engine idles terribly when cold. It ticks over lumpy, shakes the engine side to side and cuts out at will, even when power is applied. 

I checked the timing and all was good, under the rocker-cover showed no defective valves and the cam-shaft doesn't look all that worn. I guess the only thing left to check is the injectors... right?? Now hold on a minute I hear the E30 and M40 engine gurus cry, the symptoms you've described point towards an obvious massive air leak in the induction system! 

This is true and is also the very first thing I looked for and tested. Initially I performed the 'spray' test using a can of brake-cleaner (though 'Easy-start', carb-cleaner or electrical-cleaner work too, anything with alcohol content). This involves running the engine and spraying brake-cleaner around the intake-manifold, ICV, vacuum hoses, throttle-body etc. If unwanted air is being sucked in, say through a split hose, the brake-cleaner will be sucked in and combusted by the engine causing it to rev higher. If you are spraying in one particular place and the engine revs increase accordingly then you have found your air-leak. Thing is, I did this test over and over again and it showed nothing. 

I’ve heard that air can be sucked in past the injector bodies when the rubber sealing rings have degraded over time and that the lower intake-manifold is also a main culprit, but spraying round this area made no difference to the engine revs or affected the poor idle. I did a compression test and the readings were a bit low, but not enough to cause any issues.

At this point I decided it to bite the bullet and begin stripping things down. The removal of the lower intake-manifold is quite complicated because it involves removing the fuel-lines from the rail, which is why I put it off, but clearly if you want to fix something then you need to get your hands dirty so off came the fuel lines and away came the lower manifold to reveal.... well, as you can see from the photo, a virtually non existent gasket. Sigh. 

Cylinder 1 has a massive gap where the gasket is no longer there, so whenever there is a vacuum in the plenum it’s just filling up with fresh air even with the throttle-body shut and this explains why the ICV appears to be doing nothing and leads people to look there first. Whoever was refitting the manifold was obviously in a rush because the broken gasket has folded over on itself, causing the air gap to be even bigger. I guess they just tightened it hoping to make a seal, but alas no. Cylinder 2 is not much better with a noticeable gap and the rest of the gasket squashed flat. Cylinders 3 and 4 are better, but still paper thin. It looks like this may be the original gasket and the past 35 years have not been kind to it (or to me for that matter).

So... looks like I have found the culprit of the rough idle. Finally! New gasket is ordered, a snip at £6.88! Amazing how such a simple thing can cause such a severe problem and goes to show how sensitive these ‘analog’ ‘80s cars are to air metering issues!

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