Backfiring on acceleration on a 3.2

[neilschelly]

I've got an '88 Carrera 3.2 with ~180k miles, stock as far as the engine is concerned. I've had it for about 30k miles, and I don't know when the last traditional tuneup it had was prior to that. It was stuttering a bit, especially on rainy days. The car is driven every day, year-round.

I just finished a tuneup, and this was my first time doing any of this work on a 911, so it's entirely possible I did something wrong. I'd appreciate some advice on where to look now. Currently, the car starts fine, but immediately (cold or warm) starts backfiring, and not the good burble-sounding kind. It's just an occasional loud puff out the exhaust. It stutters a lot on acceleration, especially under 2000rpm. It idles up at 1100 now, and while I don't remember exactly where it was before, I feel like it was under 1000. The backfires stop when coasting over 2000rpm or decelerating, but continue and get worse whenever I'm accelerating with more than 1/3 throttle input. Overall, the engine feels down on power.

This is what I did:
* Replaced spark plugs with Bosch 4016 Platinum Plus plugs. I gapped the new ones at .028". The old ones were pretty white.
* Adjusted the valves. Every valve was too tight, so much so that I couldn't get the feeler gauge between the rocker and the rocker and the valve without first loosening it. All were adjusted so that I can get the gauge in and feel friction between the surfaces.
*Replaced the valve cover gaskets with the silicon ones available from Pelican here. I used all new nuts and washers for the valve covers, and torqued every nut to 6 ft-lbs, working from the center out and repeating that pattern several times until they were all tight.
* Replaced the fuel filter. New Mahle unit to replace the same older one.
* Replaced the spark plug wires with the set of Magnecor 8.5mm ones from this site.
* Replaced the distributor cap and rotor with a Beru cap and Bosch rotor from this site.

After two days of struggling through all this, I'm not in a good troubleshooting mindset. The most obvious answer to me is that one of the cylinders isn't firing, leading to some gas getting thrown in there and backfiring in the exhaust, leading to rougher running at lowest speeds, and more backfires as the engine speed/load increases. Does that seem like a likely reason? If so, is it safe to assume it's an ignition problem and not something I missed or mis-judged with the valve adjustment? Or is it perhaps something else entirely?

Thanks in advance!
-Neil

[neilschelly]

I opted for the post-and-then-call-a-friend approach to problem-solving, to keep myself from digging into a problem that I hadn't thought through well enough yet.

He suggested the ignition order may be off, and he was right. This was just a stupid case of swapping the 4th and 5th cylinders ignition positions. I hate how the simplest answers escape me when I'm doing something I've done a hundred times (on other cars) in a new environment (scary!).
-N

[cashman]

General consensus is to use the cheap copper plugs. Platinum plugs don't do well for some reason in these cars.

[neilschelly]

Originally Posted by cashman

General consensus is to use the cheap copper plugs. Platinum plugs don't do well for some reason in these cars.

I've still got an occasional hesitation that may be something I could blame on that, but nothing concrete yet. Once I swapped back the ignition wires, it's running right, and a bit stronger than before.

Ultimately, copper plugs spark better, but last less time. Platinum plugs aren't as good, but last a lot longer. It's no mystery - just like stickier tires won't last as long, brakes with a stronger bite probably aren't "lifetime guaranteed," and almost every other consumable on your car will be consumed faster when used. That said, I may try out copper plugs if I can't identify what's causing the occasional hesitation I still get at low speeds, but I've long suspected I've got a slight vac leak somewhere in the intake manifold, so that's the more likely culprit for now.
-N

[frannyB]

Hi Neil,

I really like the platinum plugs... My car runs smoother on them and starting is easier. I run them in the 356 and they just perform much better for me than the copper plugs. Just another data point.

Your problems seem a bit more intense than plug tech can account for. I think you are probably on the right track with a vacuum leak. Maybe you bumped an old hose? Does the idle change if you remove the oil filler cap? Can you hear a hissing when you do? Those are good quick checks for vacuum leaks.

You can double check your valves, maybe try the "Backside" method with the go/no-go set of gauges, but they would have to be really off. Maybe one loosened up on you?

I would also try putting the old plug wires back on as a test to be sure you didn't get a bad new set.

Just some thoughts...

Franny

[neilschelly]

Nah, I promise... It's running fine now. The serious issues I was describing above were the result of the reversed ignition wires for #4 and #5.
-N

[msterling]

My 88 Carrera had a vacuum leak that was caused by a loose intake manifold! I am told this a common problem with an easy fix. Just tighten the fasteners. Of course a vacuum leak could happen for other reasons. One trick is to remove the oil fill cap. If the idle speed goes down after a few seconds and then goes back up after replacing the cap then you probably don't have a major air leak. If idle speed is unchanged then there is probably a leak. This trick will not work if the leak is small. Spraying carb cleaner at suspected leak points could help to ID a leak.

[neilschelly]

I have plans to rebuild the intake manifold anyway. I want it out of the way to install a new engine sound mat, and I figure that's a good time to replace all the gaskets there, make sure all the vac hoses are solid, replace any fuel lines that look like they are as old as the car, etc. The more I hear about ethanol, and especially E15 being approved, the more I worry about getting these older fuel lines just replaced.

That said, the oil cap test is a cool idea. I think there's a small vac line that runs from the back of the throttle body over to the oil tank, so if removing that cap doesn't make a difference, it probably means your missing that line.
-N

[donagain1]

If I'm remembering previous posts properly, the issue with the platinum plugs has to do with spark intensity, as in the ability of the coil to provide enough juice to jump the gap... something which the electrinic ignition module on later models is better capable of doing.

[frannyB]

Hi Neil,

Good to hear it is all better. I did that on a Beetle years ago and it was pretty interesting

That is so awesome you are driving the car all the time! They are super fun to drive!

Franny

p.s. my theory on the plugs is that with the insulated center electrode of the Platinums, carbon can't build up. Carbon can absorb gas when it is cold causing a lean condition and then release it when warm causing a rich condition. Try wire brushing your lawnmower plug and it will start on the first pull all season. I would think that the carbon build up on the copper plugs will insulate the center electrode and also narrow the gap by its thickness, but a gap is a gap. I don't see why the Platinum would take more energy. Why would that be? Just my theory... f.