zeitgeist57 Posted August 21, 2017 Report Share Posted August 21, 2017 Car: 1995 BMW 318i 'vert - 5spd, 1.8L M42B18 with an Apexcone SS exhaust system. I've been listening to Jalopnik and a few other articles saying that it's a waste of money using higher octane gas on cars not requiring it. My CTS-V and BMW have been getting 92-93 pump gas since I've owned them, and never BP. V is rated for "premium gas" and the BMW's cap says to use "Min 89 AKI". I filled up the V one time with regular unleaded when I made a 300+mile round trip to NE Ohio and back, figuring I'd just be cruising the highway. Last time I'll do that...power was way down. However, I recently switched to 89 octane (mid-grade, never lower) on my BMW, and while there is a pretty large exhaust leak that needs to be repaired from a crack in the collector ahead of the O2 sensor, the car has always ran fine. After the switch to 89oct, once the engine is warm, and I'm entering the parking garage downtown, I notice the slightest rattle that sounds almost like knocking/detonation. I can't hear anything otherwise, because the exhaust is too loud; I only hear it when I'm rolling into the garage because of the concrete walls. Aside from knocking, can an exhaust leak or loose exhaust mimic the sound of knocking off idle? I didn't notice it before recently. It's a pre-OBDII car so I'm not sure how to diagnose if there's an actual knocking problem. Quote Link to comment Share on other sites More sharing options...
Alfa Turbo Posted August 21, 2017 Report Share Posted August 21, 2017 Exhaust leaks bouncing off of parked cars or cement will sound like all kinds of things. Fix that first. Quote Link to comment Share on other sites More sharing options...
Geeto67 Posted August 21, 2017 Report Share Posted August 21, 2017 my thoughts: 1) your BMW m42 engine is "High compression" and requires premium fuel. it can adjust to "mid-grade" and regular and will pull timing via the knock sensor to avoid detonation. If you are hearing detonation then your knock sensor has failed and you should have a code for it. You'll lose power running mid and regular but if everything is working as it should it shouldn't hurt anything long term. An easy way to spot detonation is look at your plugs. A constantly detonating (also called pre-ignition) plug will look like this: http://zooroomtechnologies.com/zr_Blog/_motorcycles/_articles/_article048images/D8X_0974.jpg 2) if you have an exhaust leak you can sometimes get "popping" in the exhaust in the over run as the hole in the pipe sucks in fresh air and helps ignite raw fuel in the hot exhaust. If you have ever followed my ducati somewhere and heard popping as I decelerated this is what I am talking about. They are tiny backfires inside the pipe. seal up the pipe. I have some high heat aluminum tape if you want a temporary fix until you can get the pipe welded. 3) a crack in the collector BEFORE the o2 sensor could be contributing to this problem as the exhaust might be sucking in fresh air when you come off the throttle and then pushing the extra air past the o2 sensor when you get back on, causing the engine to think it is leaning out and then enriching the mixture. An unnecessary rich condition can lead to some of these issues. Pull the plugs, see how they read. Seal up the hole in the exhaust and see what happens. If you still have problems, run a couple tanks of super through it and see if it improves. Quote Link to comment Share on other sites More sharing options...
mmrmnhrm Posted August 22, 2017 Report Share Posted August 22, 2017 Given the age, might a heat shield around the exhaust finally be rusting out? Quote Link to comment Share on other sites More sharing options...
zeitgeist57 Posted August 22, 2017 Author Report Share Posted August 22, 2017 (edited) It was tough to focus on a spark plug at night, but they definitely look baked! No carbon, and looked like some scorching on the electrodes and ceramic... Edited August 22, 2017 by zeitgeist57 Quote Link to comment Share on other sites More sharing options...
Geeto67 Posted August 22, 2017 Report Share Posted August 22, 2017 Ceramic is supposed to be a tan color, but the electrode is either fuzzy because of your focus or shows a little pitting due to recent detonation. I'd have to see it in person to confirm. Either way, I think the issue is less the fuel you use and more the crack just before the O2 sensor in the header. That needs to be sealed up. I know you like to be frugal with this car.....but I've never seen Walmart sell headers before: https://www.walmart.com/ip/BMW-318i-4-1-Design-Stainless-Steel-Exhaust-Header-Kit-E30-E36-M42B18-Engine/217056389?wmlspartner=wlpa&selectedSellerId=5404&adid=22222222227092267638&wmlspartner=wmtlabs&wl0=&wl1=g&wl2=t&wl3=213302863891&wl4=pla-536685449032&wl5=9014871&wl6=&wl7=&wl8=&wl9=pla&wl10=114221067&wl11=online&wl12=217056389&wl13=&veh=sem Amazon has one for $10 cheaper: https://www.amazon.com/Design-Stainless-Exhaust-Header-Ceramic/dp/B00NI73F3Q Considering all the junkyard ones are probably equally as cracked and this is your second cracked manifold on the car, might be your best bet questionable quality and all. Quote Link to comment Share on other sites More sharing options...
zeitgeist57 Posted August 22, 2017 Author Report Share Posted August 22, 2017 You're right, Kerry...the collector definitely needs to be repaired. However, if the crack on the collector is upstream from the O2 sensor, wouldn't it be reading lean - either from the introduction of outside air or escape of unburnt fuel through the crack - cause the engine to run rich? I've definitely kept an eye on E36 318 headers...but the mixed-bag of responses (at best) makes me question investing $90 in a "stainless steel header" that still: 1) Requires exhaust studs in the head. If one of the old ones breaks off in the head when I'm removing the factory manifold....it'll be a lot of unnecessary work and cost to extract. Plus I still have to get the new hardware. 2) Good number of stories about the welds on chinaBay headers leaking from the factory. Dumb to ditch a cracked factory pipe for a cracked china joint. 3) Dubious fitment - would suck to have to cut/weld a brand new header. 4) I toyed with the idea of getting rid of the cat, but I'm getting old...I still like my street cars low and loud, just not stinky. http://www.ebay.com/itm/STAINLESS-STEEL-4-1-HEADER-FOR-89-96-BMW-E30-E36-318-Z3-4CYL-EXHAUST-MANIFOLD-/391867187181?hash=item5b3d1abfed:g:jUMAAOSwruhZjkHI&vxp=mtr I have a friend (you know the guy) offering use of a TIG or MIG and a lift to fix the collector. I MIG'd it before with my HF flux-core...yeeeeeeeaaaaahhhhh, that didn't seal. I'll try to seal and re-gasket the collector and reinstall at the header first to see if that lasts before going eBay/Amazon... Quote Link to comment Share on other sites More sharing options...
Green Bastard Posted August 23, 2017 Report Share Posted August 23, 2017 Is the head on this car aluminum? If so, broken studs wouldn't be that big of a deal imo. If you break one, weld a nut on it and turn it out. Quote Link to comment Share on other sites More sharing options...
zeitgeist57 Posted August 23, 2017 Author Report Share Posted August 23, 2017 Is the head on this car aluminum? If so, broken studs wouldn't be that big of a deal imo. If you break one, weld a nut on it and turn it out. Aluminum head, yes. Good to know. When I had the head off last time, the studs all came out with no issue which surprised me. Still, with rusting and heat cycling, I worry... I'll TIG weld the collector first...see if that holds. Quote Link to comment Share on other sites More sharing options...
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