Sooty exhaust at high RPM

I passed the emissions test easily but failed because there was too much visible smoke at high RPM (I’d think that would redound upon emissions - wrong again.) The guy was sure I just needed new plugs and wires, so I bought them and an air filter. I replaced the fuel pump and filter the day before, the oil and oil filter the day before that, the PCV valve last week - have I missed something?

He’s right: the exhaust at high speed is sooty. That’s not the kind of thing I look out for, but after I installed the new plugs and wires and air filter, I started it up and pushed the pedal all the way down: I left a big spot of soot in the pavement.

I got a new muffler and tail pipe 4 years ago; he passed the catalytic converter. Everything under the hood looks okay: no drips, no broken wires or hoses or connections. It’s rated at 700 idle speed, but stalls out that low since I put the new fuel pump in. Could a wrong fuel pump cause this symptom?

Simple question.

Are you running E10 gasoline in this 87 pickup?


It’s seasonal in Albuquerque, so sometimes. Probably always when I lived in California. I’ve passed a dozen smog tests. I don’t accelerate that hard when sitting still and don’t see my exhaust when I’m driving so I’ve never noticed this before.

BTW, the old plugs look okay. Haynes and Chilton have color inserts with pictures of problem plugs; I saw none of that on my old plugs.


It may be running very rich, especially with wide open throttle. One cause is a broken AAP (Auxiliary Accelerator Pump) diaphragm. Leaking, it lets raw gas be pulled into the base of the carburetor. After replacing a couple of them I disabled the whole AAP system on my 1979 Toyota truck’s 20R engine and it still ran fine. I don’t remember just how I did it. One way is to plug the rubber hose between the AAP housing on the side of the carb and the little metal pipe it runs to, at the base of the carburetor. Plug it with a BB or a piece of a nail.


Would this cause backfire? My choke broke 17 years ago, resulted in running rich and backfire. I don’t have backfire now.

I rebuilt the carburetor 4 years ago, put in a new AAP. I’ll try this. It’s only supposed to work on start, right?

I’ve never done this test before. Would most cars not leave a sooty stain on the pavement if you pressed the accelerator all the way down for 10 seconds? My exhaust is on the driver’s side, so I could stand outside and watch with my foot on the pedal; it didn’t look that smoky to me, not as bad as those coal-rolling jerks.

Backfiring through the carb is a sign of lean mixture. Soot is a sign of a rich mixture.

E10 will naturally run leaner than E0 gasoline. E10 fuel can also ceeate deposits that clog air bleeds to cause that rich mixture as well as attack rubber parts never intended to use ethanol laden fuel.

I think @shanonia is on the right track and you should try that first. Next step might be cleaning and rebuilding the carb.

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It’s 34 years old, been burning it all that time, passed smog tests all that time. It tested okay 2 years ago, and that was only 500 miles ago. I re-built the carb 4 years ago.

Could a faulty fuel pump cause this? With the old fuel pump it idled okay at 700 (book); I had to bump it up to 1000 to keep from stalling out when stopped with the new fuel pump - and the old one was leaking.

Running an excessively rich mixture will cause black smoke from the tail pipe. Since your truck passed the exhaust gas sample test you don’t have an excessively rich fuel /air mixture.

The purpose of the tail pipe inspection at idle and 2500 RPMs is to identify oil burners/smoking vehicles. These polluters can pass the exhaust gas test but they produce excessive pollution.

Most cars would not emit soot under that condition, your exhaust system may be loaded with deposits from years of short trips and conservative driving. You should not operate the engine with no load at full throttle.

Operate the engine at a steady 2500 RPMs for several minutes and observe the exhaust to see if your truck smokes.


When I changed the oil I put in all 5 quarts; the specs call for 4.5. The dipstick reads a little above full: could that cause oil-burning? I’ve done it before: the crankcase doesn’t overflow when I put in 5 quarts, and it hasn’t filled the new filter has it?

Thassa me! I got a new muffler and tailpipe 4 years (=600 miles) ago, as well as re-built my carburetor. Isn’t that most of the exhaust system?

It’s what the emissions-testers do. I was copying them. I’ve never done it before.

It’s operated funny since I put in the new fuel pump. I asked about that in another thread, which attracted no interest (which I should probably interpret). Is there any way a wrong new fuel pump could be involved? I ask because I can return this one and try the other, which is the same model as the original and first replacement.

Highly unlikely

Yes, but…

Consider a garden hose that’s full of poison. You replace the nozzle and 75% of the hose. Would you drink water from that hose? :wink:

You replaced most of the exhaust, but there’s stuff upstream of what you replaced including the header that still has deposits in it.

Have you Seafoamed it yet? I ended up doing two Seafoam treatments on my '88 truck and it made a lot of difference, including eliminating the giant black smokescreen the thing would put out when I started it.

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Yes. As I recall, once the engine is warm the AAP system stops enriching the mixture. I don’t remember just how. But if the diaphragm is broken, gas can be sucked into that metal pipe all the time, especially when accelerating.

I am kinda stumped on the problem that coincided with the new fuel pump. Maybe just a…coincidence. I would get that AAP shut off and see how it goes.

Did you use a rebuild kit that had ethanol compatible components?


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I wouldn’t consider an 87 Toyota a classic car. And E-10 has been around everywhere I lived before that vehicle was every built. I seriously doubt E-10 or not is going to make a difference.

Unfortunately that’s meaningless, and I don’t mean what you said, but the term “classic car.” The Classic Car Club of America doesn’t consider anything newer than 1948 to be a classic - something I suspect @Mustangman would consider outright heresy. :wink:

On the other hand the Antique Automobile Club of America thinks anything that was mass-marketed and built 25 years ago or more is a classic, which would qualify that '87 Toyota.

Meanwhile my neighbor is pretty convinced his 2010 diesel Jetta is a classic. He’s wrong but we can’t prove it.


Nor would I.

But I do consider it as a pre-etanol vehicle.

That’s a vehicle that was manufactured with fuel components that aren’t compatible with ethanol gas.

Minnesota mandated E10 gasoline as the main motor fuel in 1992.

So, we have a pretty good grasp of what ethanol gas can do to a non-ethanol fuel system.


E10 was in common use in the Southwest in the 1980’s.
Even if Toyota was not up to date with their vehicles, all of the rubber fuel system parts on this vehicle have been replaced at least twice.

Did the use of E10 during the last 30 years cause the engine wear that is causing the smoke? Doesn’t really matter. Perhaps new valve seals and a 20 mile highway drive will get this truck to pass inspection.


But it wasn’t government mandated like it is today.


Ethanol wasn’t mandated like it is today…however it’s been around in many parts of the country long before it was mandated, and before this vehicle was ever built. I never heard of any problems with any vehicle in the area’s I lived that had engine problems due to gas used.