Spark plug gap: 16 years in error

[RobertRO]

As of now, I can count about 16 years of owning a BMW M30 powered car. First it was an E28 535i with the M30B34KA (cat) engine, and currently I have an E32 730i with the M30B30 engine (well, actually a hybrid with M30B35 block and M30B30 head).

For all this long time I have been plagued by an annoying issue. It usually took quite a long cranking time (usually 2-3 seconds, but sometimes reaching 4-5 seconds) to start the engine when hot. In my numerous attempts to solve it, I have been checking and/or replacing all the fueling and ignition components. To no avail.

A recent experiment (ran for a totally different purpose, namely playing with a Microsquirt based controller) revealed a mistake that I have been making for 16 years.
I have always used NGK ZGR5A on my M30 engines. And I have always installed them gapped as supplied by HGK, which is 0.8mm nominal, but actually a tad on the loose end of it. This means that the gap was closer to 0.85mm for a new spark plug, and reached about 0.90mm toward the end of spark plug lifetime.

I was stunned to discover that, by setting the spark plug gap at 0.75mm (0.03"), the M30 engine starts right away even when it's hot.
It is unbelievable the huge impact, in a positive way, of a mere 0.1mm less spark plug gap.

I guess you don't mind my sharing this with you...

[SeattleGuy]

Interesting. My car started immediately hot or cold but actually after the EAT chip I have to crank it twice when totally cold sometimes before it catches. My plugs are a year old and set to the larger side of gap.

[Federico]

That's what happens when you don't read the manual
BMW would specify the gap for, say a 535is, to be 0.7 + 0.1 mm
Note that's "+", not "+/-". I've always set them to the lower end and if I check them later and see them at 0.8, I know they'll need to be replaced soon.
I have regapped worn plugs back to 0.7 and the engine does get some ooomph back, but it won't last more than a few hundred Km's. Its just to get by and at least it's not a horrible job to do on an M30.

[Blue Shadow]

A&E motorsports recommends a .032 gap for performance, the wide side of the allowed gap.

Interesting that the cold start was quick with the closer to spec gap. I had great bump the starter and the car is running until I swapped in the 5.0 injectors. But that cleared up the slow hot start issue and isn't too slow to be acceptable.

[SeattleGuy]

A&E motorsports recommends a .032 gap for performance, the wide side of the allowed gap.

Interesting that the cold start was quick with the closer to spec gap. I had great bump the starter and the car is running until I swapped in the 5.0 injectors. But that cleared up the slow hot start issue and isn't too slow to be acceptable.

Yah I went about .030-.032 .... I'm gonna swap some newbies in at .028 and take a look-see.

[Mike W.]

I think the ECU is at least a, if not the, determinate on how fast it can start. When I put L jet on my Bav, it was a coworker who first pointed out it started on the 6th compression cycle. It only had a 1.0 PS starter so it was easier to count. On my 535i E28 it seemed like it would start on the 3rd compression cycle, which makes sense because it had to see the reference sensor at least once, maybe twice and I could only count 3 but it was really 4, I dunno. Now maybe a smaller gap would ignite easier for better starting, but unlike say a carb with a shot of the accelerator pump I think the electronics are a limitation. Both of the above were consistent and repeatable.

A curious sidebar is other relatively new cars don't seem to start as quickly as my E36/E39 cars. Not slow to start, but noticeably slower. You have to hold the key in position for, I don't know, maybe 2 seconds until it fires instead of almost just moving it to the start position momentarily.

[SeattleGuy]

A curious sidebar is other relatively new cars don't seem to start as quickly as my E36/E39 cars. Not slow to start, but noticeably slower. You have to hold the key in position for, I don't know, maybe 2 seconds until it fires instead of almost just moving it to the start position momentarily.

Yup. That there is the government monitoring all of your vital signs to figure out what your flesh is really worth. Check it out.
Hahahaaa... sort of. In Time...... What a fucked-up movie, but then again you know as with Enema of the State they are basically forecasting what we are going to go through. Oh yes.
https://www.youtube.com/watch?v=y9DgUkY ... %E1%BB%81u

[BMWCCA2]

A curious sidebar is other relatively new cars don't seem to start as quickly as my E36/E39 cars. Not slow to start, but noticeably slower. You have to hold the key in position for, I don't know, maybe 2 seconds until it fires instead of almost just moving it to the start position momentarily.

Most newer cars, especially BMWs for over a decade, have a start button that you don't even hold. It just does its start cycle—just push once, momentary contact, and it starts.

I remember the sticker on my '73 2002tii windshield that (in German) warned about it taking quite some cranking to start. No big deal. Price of admission to the early days of fuel injection.

[gadget73]

sequential FI that needs to "see" whatever indicates cylinder #1 does typically need to rotate enough to pick that up. Batch fire I wouldn't figure makes any difference since half or all of the injectors fire at once anyway.