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Hello, this is my first post here, I'm just looking for some troubleshooting advice:

My car is old, about 216,000 miles, and one of the few carburetters left on the road (an 89 Corolla). For about two years, it's been having a misfire problem. It seems to be fairly random, but generally it seems to be most likely to occur when the throttle position is low, irrespective of gear and speed. (Actually I mean when throttle position is moderate, since my throttle position is never "low" except when I'm in the clutch or neutral. :) )

The misfire is fairly mild, but definite. Also it's hurting my mileage! My methods usually yield around 45-50 mpg on my commute when I drive my 97 Civic (per the Scangauge trip meter). When I started hypermiling two years ago I initially was getting 40-50 mpg in the Corolla, but within three tanks the problem started and my mileage dropped to about 35 mpg average, and I've never had a tank over 42 mpg since then. :(

I've tried a lot of things to fix this, but so far to no avail:

Had a shop tune the carb.
Replaced distributor cap/rotor and spark wires, but not the rest of the distributor.
Check sparks and gaps - no problems.
Replaced fuel filter.


So here are the ideas I've got left: (help!)

Switch back to non-synthetic oil? The problem did seem to coincide with my switch to synth, but you know my compression numbers are perfect which makes me wonder how on earth oil type could affect firing... I don't get it...
Replace the rest of the distributor?
Ask the shop to tune the carb again?
Replace/rebuild the carb?
Stop using high octane? (Now I'm grasping at straws - I switched years ago thinking I had some preignition...)
Advance or retard timing? (Right now it's dead on the VECI label and the shop agrees.)


Anyone ever had a problem like this? Is it possible for oil type to be a factor?

Thanks,
Chris

Check for cracks in the distributor. That can let more than enough moisture in to start sending electrons to the wrong place.

Change your spark plugs. My car was misfriing and I thought the plugs looked ok and I was scratching my head and took to a shop. they called next day and said was the plugs.

Coil? Especially if the misfire is noted when the engine is very cold or very hot or in wet/humid weather?

You are getting some good ideas.

With that many miles, the distributor bearings, advance mechanism or other parts could be worn causing timing jitter. If it misses at moderate RPM without load, you could look for timing jitter with a timing light. Also suspect the distributor condenser.

Sometimes there are various transition jetting in the carb. There could be some trash, or misadjustment, but the carb overhaul should have fixed that, if done by a professional.

You might look for loose rubber hoses for various things under the hood, like the vacuum advance hose. Hoses deteriorate, get loose, get brittle and get holes. How about the EGR valve?

Sometimes the brake booster diaphragm gets a hole, causing loss of vacuum. That might show in a strange brake feel. If you suspect this, you can plug the manifold fitting, realizing that you will have no power brakes while you test.

You might look at the vacuum with a vacuum gauge. If you don't have one, you can probably borrow one. You can also run a compression check, if you can find a gauge. (Ops, see you did that.) I don't recommend "throwing parts" at this kind of problem. Someone should be able to pinpoint something.

If it only misses under load the problem is a bit more difficult, since testing is more difficult. A good shop with a chassis dyno could probably reproduce the miss and likely find it, but sometimes that gets expensive. An hour of dyno time can easily run $150. A really good old line mechanic might find the problem pretty quickly, if you can find one any more.

One last question, are you sure the engine is carbureted? I did a bit of searching on Northern Auto Parts and I see fuel injection parts, but online catalogs aren't very reliable on old stuff.

To answer a couple of questions:

It doesn't seem to be related to temperature, engine warm-up, or weather. The only correlation I've noticed is throttle position (or maybe load). Higher throttle position or higher load -> less likely to misfire in a way that I notice. (Now that I think about it, it's probably better to call it load. So it happens under light load, not idle, and not heavy load.) It's also possible that it misfires under higher load but it becomes unnoticeable for some reason.

I haven't noticed any change in the feel of the brakes, and I've been in the engine and haven't noticed any bad looking hoses or hissing sounds - not like that proves anything though.

The carb was "cleaned and tuned" but not rebuilt.

I can tell you that the timing doesn't jump when it's idle. I'd need the shop to do that test under any load.

It is definitely carburetted. That wasn't the most common engine on the 89 Corolla, but mine is a pretty stripped-down base model, no power windows, no power steering, no AC (except for an aftermarket unit that has never been hooked up while I've owned it), so maybe the carburetted engine was NOS they put in on to pinch pennies.

Another clue I remembered: in SLC we get an emissions check every year. My car passes easily, but there was a definite increase in emissions when this started. Especially I remember the CO and HC- were high and O2 was lower than normal. To me this sounds like a rich mixture, but then I'm not really familiar with the test conditions and I don't know if this really tells us anything new.

I might not have time to tinker with this right away, but there are definitely some good suggestions here that I haven't tried yet. Thanks for your help!

I had an old car once that had the other half of the distributor go bad, but it didn't show up with your symptoms--nobody figured it out until they put it on a 'scope. Sometimes little air leaks show up as little misses at low throttle. Little air leaks can be compensated to some extent by tuning the carb, so maybe the previous tune up should've taken care of that?

I've never wrenched on a Corolla so I have no idea where to start checking for leaks.

I realized there are some more clues that seem pretty important (yes, I still have the misfire...):
* Sometimes it does idle rough, if I wait until it does, then unplug sparks one at a time, cylinder 1 is insensitive (indicating it's probably at fault for the rough idle).
* The plugs in cylinders 2-4 have a light white deposit on them (too hot?)
* The plug in cylinder 1 has a black powdery deposit (carbon)
* The emissions test is at two speeds, at low speed the emission is much much worse. (I think low speed means low RPM and low throttle position.)

To my way of thinking, this eliminates the carb and probably the coil (unless maybe a weak coil could be a contributing factor...?) A priori the EGR could possibly be at fault if for example it were located right next to cylinder 1, however I found the EGR valve on the opposite side of the intake manifold, next to cylinder 4, so I think that rules out the EGR as well. I think that leaves me with the distributor, plugs, or spark wires, and given what I've already replaced that points towards the distributor bearings, or else a vacuum problem around there, unless for example the replacement distributor cap is also faulty.

Any thoughts? I'm thinking a scope on the ignition could be very helpful at this point. (Although I have no experience with that - I need to choose between learning how to do it myself or finding someone who does.)

It probably wouldn't hurt for me to spend some time checking my vacuum system (being as old as it is) and also the EGR operation. My plugs remind me that I've been running high octane gas for years to try to reduce preignition, which may indicate a separate EGR problem.

Thanks for the help,
I'll get the thing fixed... eventually...

Plugs 2-4 may be showing that they are running too lean. (Though that may be what is intended for good MPG. And I have heard from some old-timers that reading the plugs isn't that easy with today's lead-free oxygenated fuels.)

It sounds like cylinder #1 is either running rich, or has oil fouling. It sounds like you have a single carburetor, so one cylinder being rich is a very low probability, unless it is a common problem for these cars. (E.g., the manifold design is poor so you tend to get more fuel in #1 than the others.) If the soot on the plug is sticky, that is definitely oil. If not, it might be excess fuel...

How's the compression? Looks like you said it was good, so it's less likely to be ring issues. Might be a valve guide? Is there any smoke?

If you clean off the fouled plug or replace it, do things get better for a while?

Misfires of any kind can cause the high HC readings.

-soD

Guess what? I rechecked the cylinder 1 plug, and it's changed. Now it looks almost like the others. I last looked at it three weeks ago. Two weeks ago I switched from 10W-30 synthetic to 10W-40 synthetic blend 'for high mileage engines'. I thought that my engine ran better afterwards, but it does still miss sometimes.

(So I can't say whether the black deposit was sticky, although my foggy memory thinks not. I remember it being powdery. The engine holds oil pretty well, I lose less than a half quart in a 4,000 mile year.)

I won't have mileage numbers for several more weeks, but maybe this means I have an oil leak on the valve side, and oil weight affects it. Maybe the 'high mileage' additives do too.