Engine constant miss?

Lock · Post by **Lock** » Tue Dec 12, 2017 1:17 am

Hi guys, wondering if anyone can help. My engine has a constant miss which causes the idle to be very shaky, it's had it the whole time I've had the car (2 years now).

You can still hear the miss around 2000rpm and there's the odd bump but it drops away and higher RPM doesn't sound or feel like its missing at all. When driving the engine is very powerful, revs easily and pulls strongly all throughout the rev range. But when it idles misses constantly and feels (and sounds) like a bucket of bolts especially because I have a dual-mass flywheel with the Getrag 260 which has a bit of wear on it, so every miss causes a metallic clatter. I'm nearly out of ideas and I don't want to keep replacing parts just guessing. Let me know if you have any more ideas of things I can do.

When the car is in the garage and I go into my lougeroom I can hear the miss as a burbling sound. It sounds like one or two cylinders miss a couple of times in a row, then fires OK for 2-4x that time, then a few misses in a row again, repeat. I definitely couldn't balance a soda can on the engine, it would fall off immediately.

So far, the things I've tried, with no change;

Replaced the distributor cap, rotor, plug leads, plugs with Bosch copper WR9DC+
Tested bypassing the coil resistor (for stronger spark)
Adjusted the spark plug gap from 0.7mm to 0.6mm "just in case"
Adjusted valves at the lobe side - three times, last time slightly bigger gap
Adjusted timing to various settings, currently at factory spec ~22BTDC
Replaced fuel injectors with OEM re-manufactured ones
Replaced high-pressure side fuel lines, fuel filter
Installed fuel pressure gauge, pressure is according to spec, regulator working correctly
Checked for vacuum leaks all over the engine, replaced all under-hood vac hoses including evap
Cleaned ground wires on engine
Cleaned/adjusted the throttle and throttle switches according to the FAQ (and yes the switches are wired the right way around)
Ran Seafoam through the intake and also in the engine oil

What I haven't done:
I haven't replaced/swapped out the fuel pump or coil ($$)
I haven't run a compression test
I haven't inspected inside the engine with a little camera to stick in the spark plug holes. I've bought one - just don't know what I'd be looking for

Info that might be useful - 1979 M30B28 with L-jet and oxygen sensor. The head was replaced at some point with an AMC head. The AFM is a reconditioned unit. The battery is in average condition but could be replaced soon. The coolant level doesn't drop, it doesn't burn oil, and it passes California smog with flying colors. Idle set at around 1000 (above the suggested 800) because the slower it turns, the worse the miss is.

It's not lacking in power and it's smooth to rev out to ~6000rpm. Perhaps there is internal damage. Or maybe I am asking too much from a 40 year old engine. I can try and upload some video if that would help.

Let me know what else I can try!

T.Hanson · Post by **T.Hanson** » Tue Dec 12, 2017 7:11 am

Before the experts make suggestions, Autohauz sold me an oem fuel pump for $100. Not the national debt after possible 30 yrs. and Mike saying they get tired.

Picky, but I thought the factory spec idle was 1200 r.p.m..

I read the in tank fuel pump, filter screen foot can get dirty, and the return fuel line from the exterior pump should be in new condition. There is a recent post for reading how to replace it with the least hassle.

Lock · Post by **Lock** » Tue Dec 12, 2017 12:02 pm

Yes actually I was going to post a reply to you about that, I just kept planning on this post. In my book and on my engine bay sticker it says the standard idle is 800rpm. I figured maybe you had a 530i which have a different setup with their emission stiuff?

I will add "check fuel tank filter" to the list, thanks.

I did install a fuel pressure gauge, and it reports ~40psi which is according to spec, but lower engine speeds (lower alternator output) might account for a slower speed issue like failing fuel pump or coil. You're right $100 isn't bad for a pump and it's $50 for a coil. Afters spending about $150 on the injectors (and they are a pain to install) I'm just wary of throwing more parts at it, that's all.

maybeillbuyit · Post by **maybeillbuyit** » Sat Dec 16, 2017 12:51 pm

That sounds like an annoying situation. I've always had cars that just won't idle properly and I know it sucks. I assume you've tried pulling the spark plug leads one at a time while idling to narrow down which cylinder is missing? And looked at the plugs for any differences? How about switching out the ECU for a known working unit. Its good to have a spare either way.

Not sure if the 79 had the transistorized ignition module that hangs on the pass side fender?

Lock · Post by **Lock** » Mon Dec 18, 2017 1:28 am

Thanks for your reply. Yes, it's mostly only annoying at this point because it has plenty of power and loves to rev especially into freeway onramps, just can't idle well and is embarrassing when I'm trying to show it to people because that and the bad paint make people think it's just some old junker. But it starts every time and has never let me down.

I don't have any spare parts (in good or otherwise condition) like an ECU, coil, fuel pump etc. But I just ordered a compression testing kit and also an ignition line tester that goes between the spark plug wires and plug itself, I'm hoping to be able to 'see' the miss - is it before the spark plug or after. I'll pull each line one at a time as well to see if I can narrow down which cylinder it is - is it the same or different ones. I re-gapped the plugs and looked at each one, I'm no expert but they all looked the same and didn't look damaged, they're only about a year old.

The BMW has been banished outside to the driveway this week for being annoying (also so that I can give my DD some care), but next week I'll bring it in and run some more tests.

Yes this one has the transistorized ignition module, I'm not sure how to test its health but I've cleaned the plugs and checked the supply voltages which all seem OK to the best of my ability.

Merlin · Post by **Merlin** » Thu Dec 28, 2017 3:09 pm

Ok time to fix this thing... when you checked for vacuumed leaks did you check the paper gaskets? Where you using a spray like brake clean? When u use spray don’t go across the positive poll on the starter.

I want you to check the color of your spark on all the spark plug leads. Grab a good clean spare spark plug. Pull the fuse for the fuel pump. Do NOT hold the plug wire or the plug during the test!! Plug the spark plug in and rest it on the valve cover to ground it. Have an assistant spin the engine over and check the color of the spark. It should be white to blue. If it’s good, then job done it’s not in the electrical side.

What do I think it is, the AFM, get a spare one (used from somone like Al Taylor) he is cheap. The sweep arms can degrade over time or get corroded, just because it was rebuilt doesn’t mean it’s not the problem. Another way to test the afm is to tap it while running, if the miss gets worse then bingo. You can also flip them 180 degrees and if it won’t run or is worse, bingo.

Good luck

Lock · Post by **Lock** » Tue Jan 09, 2018 2:33 am

Thanks for your feedback. I had some time this weekend to run a bunch of tests, and this is what I found;
Compression test returned about 160PSI on each cylinder cold with 6 cranks each time. Tells me both the top end and bottom end are good
Spark is bright white/blue and can jump a far bigger gap than just the spark plug when the engine is running (had a lead tester and electrician gloves) so the coil seems fine, plus the coil resistance is within spec
The distributor reluctor pickup coil produces the correct voltage at the elec ignition unit plug according to spec (~0.05v)
Tapped the AFM, no change. I took off the intake boot and nudged the flap with a screwdriver while running, and also opened the throttle and nudged it, no change to the miss so I believe the AFM tracks are okay and it's alright. I noticed that at idle, if I move the AFM flap the engine idle speed increases to a point - so the engine is normally running lean at idle, but that's possibly by design though. This also tells me that it's not a vacuum leak because if I richen up the mixture to compensate it should reduce the miss, right?
Which paper gaskets did you mean? I recently took off the 'bananas' to replace the fuel injectors and replaced gaskets there as needed but absolutely no change. Plus as mentioned I richened up the mixture which had zero effect. I've sprayed carby cleaner all over and didn't have an effect, although I've never looked for vacuum leaks before so I might have been doing it wrong.
I'm no mechanic, but I feel like it's in the low tension side, somehow it's just skipping some of the pulses. Is it possible for the electronic ignition unit to go intermittent? I opened it up (as much as you can) and it 'looks' okay, but I don't know how to open it fully without cracking the case, I'd like to re-flow the solder joints. They are $300+ new so I'm not buying one just on a hunch. What I will do is try and bypass the lead from the distributor to the ignition module. The wiring is 40 years old the low voltage lines (0.05v) might easily be getting lost.
Might be totally wrong but man I can't figure out where the problem is and it's driving me nuts.

Mike W. · Post by **Mike W.** » Tue Jan 09, 2018 1:48 pm

Since it passes smog with no issues I wonder if it really is a miss. That almost always fails them. The idle on these cars was never that great. It's best described as hunting, but kind of seems like a miss. Try this, unplug the O2 sensor and see how it runs. If it smooths out it might just be the way it's supposed to be, disconnecting it will richen it up a bit but also stop the feedback which drives it up and down leading to that irregular idle. Also connect the O2 sensor to a voltmeter while in operation and compare the readings to how it runs. Now it might be a miss, I really can't tell from here, but it might not be.

If it really is a miss, and pretty consistent, try idling it for an extended period of time. Then shut down and check the plugs, where ever the miss is should look different.

Lock · Post by **Lock** » Wed Jan 10, 2018 11:42 pm

Thanks Mike, as our cars are so old I have no reference for what a normal idle is like. One thing forgot to mention is I found last weekend the distributor centrifugal advance wasn't really retracting, I oiled it up and now it does work - but it's messed up the timing now that it functions. Once I've re-set the timing I will take a video and try and link it here, see if you guys think that looks/sounds normal or not. My friends all have e30s and they idle without a hint of shake.
Also I'll try unplugging the O2 sensor and see what happens. My idle speed doesn't really go up and down it's always very fixed except for the shuddering. I bought a new O2 sensor to pass smog three years ago so maybe time for a replacement anyway.
The miss makes a distinct burble sound in the exhaust if you listen carefully, and I can still hear it as I rev the engine up to 2-3k but I can't hear it after that due to too much noise. It's so erratic I can't tell by the sounds or the shakes if it's the same cylinder or not. Maybe I'm just nuts.
Don't know if it helps, but here's the 1+ year old spark plugs - they all look the same, with an almost green/white color.

Mike W. · Post by **Mike W.** » Fri Jan 12, 2018 1:20 pm

Those plugs look great. I can't tell from here what it sounds like, but due to the O2 input and engine management the idle at best is nowhere near silky smooth. It's always going rich then back to lean, then back to rich... again and again. And not by accident, it was supposed to. Hunting or uneven are a couple of words that have been used to describe it. With all you've already put into it, before anymore, you might want to take it somewhere with a smog machine and see what it's doing at idle, just idle. The dynos in use now flunk a lot more cars, but pass some that wouldn't have passed on the unloaded test. Idle can be different, but I'm guessing if it passes smog it's ok.

Lock · Post by **Lock** » Sat Jan 13, 2018 11:54 pm

I took a video of the idle with a jug of windscreen fluid balanced on the manifold, seemed the best way to visually show the shake.
https://flic.kr/p/Fjp2gu
The metallic rattling sound is just the e34 dual mass flywheel that's a little worn but still serviceable. The bad idle emphasizes it though. But every knocking sound translates as a shudder through the steering wheel.
Engine at operating temperature, idle speed around 900-1000rpm, everything plugged in as normal.
I unplugged the O2 sensor, the idle went up a bit, and the voltage hovered around 0.8-0.9 volts which seems to be the right voltage if the engine went rich when it was disconnected from the ECU.
Am I able to go to a smog shop and ask him to run a non-DMV test, or would this have to be a garage with say a CO analyzer/other smog test equipment?

T.Hanson · Post by **T.Hanson** » Fri Jan 19, 2018 12:05 pm

A month below zero on the tundra, staying in the heated box reading the posts. Familiar with the," Never give up," syndrome, the problem thingy is in here somewhere, find it and win !

No longer able to keep my suggestions untyped. To minimize the risk of winning at a cost able to afford a fully restored Ferrari, buy a rust free, nice running example. Use the parts one by one, swapping in to note the effect. This in tandem with critical, close up inspection of every tiny connection, inch of wiring, fuse, relay,... perhaps as guided by Mike as most likely.

Having two has worked for me, as has assembling a pile of parts from the three that returned to rust dust.

T.Hanson · Post by **T.Hanson** » Fri Jan 19, 2018 1:42 pm

Or this :

https://grassrootsmotorsports.com/artic ... n=GRMDaily

Lock · Post by **Lock** » Fri Jan 19, 2018 8:45 pm

T.Hanson wrote:A month below zero on the tundra, staying in the heated box reading the posts. Familiar with the," Never give up," syndrome, the problem thingy is in here somewhere, find it and win !

No longer able to keep my suggestions untyped. To minimize the risk of winning at a cost able to afford a fully restored Ferrari, buy a rust free, nice running example. Use the parts one by one, swapping in to note the effect. This in tandem with critical, close up inspection of every tiny connection, inch of wiring, fuse, relay,... perhaps as guided by Mike as most likely.

Having two has worked for me, as has assembling a pile of parts from the three that returned to rust dust.

I always keep an eye out on Craigslist and ebay for cheap parts. Seeing the ignition module for sale for $400+, you can imagine I was pretty happy when I found a guy in the UK selling a NOS one for $30. I snapped that up quick. The coil has tested okay and the spark is powerful, so if the swapped in ignition module doesn't fix it, then I will try that RockAuto distributor rebuild that's mentioned in one of the other posts here.

I have stopped every other project on the car until this is fixed, which is motivating. But if it's too much trouble or I get tired of it, I will skip ahead a few projects and move right on to replacing the engine with an M30B35 which I was holding off until a reasonable time but maybe that time has come.