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Tinkering Addict
89 Tercel EZ
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Discussion Starter · #1 · (Edited)
Intermittent stuttering under load - 89 Tercel

I'm having a problem where the engine misbehaves under extreme load, especially going above 60MPH up a steep mountain. It's as though suddenly either my timing or fuel/air mix is very wrong. I get black smoke behind me and shuddering and the car slows down when I press the gas harder and it goes back to normal when I ease up on the throttle. Everything works fine under normal load conditions and the car has plenty of power except when this happens. The problem is intermittent, but only happens under really heavy engine load.

I suspect that the ECU is dropping out of closed loop under steady heavy load because of a missing in-line vacuum delay or check valve that used to be attached. I put a delay valve in the place where the missing thing should go for the purposes of this picture, but I don't remember what was originally there. Anyone know what goes there or have another idea of what might be causing my problem? See the link in my signature to see what my current configuration is...


BTW: I have a Weber 32/36 carb, not the stock one. I already replaced the plugs and wires, cap and rotor w/ NGK parts and tested the ignition coil resistance. I've also disconnected and plugged the EGR's vacuum hose and tried removing delay valves from my vacuum advance, taken the temperature controlled vacuum switches out of the loop - all to no effect on the problem. Running out of ideas...
 

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Tinkering Addict
89 Tercel EZ
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332 Posts
Discussion Starter · #2 ·
Well I think I can rule out the ECU going in and out of closed loop - the problem happens with it disabled altogether. Would still like to know what goes in-line with the vacuum sensor if anyone with a stock setup on a 87-90 carb Tercel wouldn't mind looking and maybe taking a picture for me.

I found some posts by others who blame severe carbon buildup for this sort of problem, so I ran some sea foam through the intake today and it belched smoke for 20 minutes and fogged up the neighborhood pretty badly... That seemed to help the car run smoother in general, but I don't know if it solved the problem altogether since it only happens sometimes. Crossing fingers...
 

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black smoke is sometimes a product of incomplete combustion of the fuel with more load the fuel might not be burned fully causing the lack of power and smoke id check the fuel delivery and plugs, im not to sure i dont know much about carburated vehicles
 

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Tinkering Addict
89 Tercel EZ
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332 Posts
Discussion Starter · #4 ·
Old plugs looked normal and were only 6 months old. Bad plugs and wires were my first thought when it started having problem. I changed plugs and wires, but the problem didn't go away. Fuel delivery system is almost all new except for the fuel pump, which seems to crank out plenty of fuel.

The strange thing is that it had been running better than ever recently... until I drove for about 3 hours on the freeway trying to go on vacation and the car just started stuttering and struggling to go up the hills at high speed. Now it does this often on the highway but not at slower speeds.

What's really strange is that my EGR's vacuum switch and my MP3 player failed within ten minutes of the problem starting. Maybe the car just needs an exorcism or something, I dunno...
 

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... It still runs...
Tercel
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Im not realy an expert at all so...

well my relative has a MG car its carburated too, and i used to have radio controlled airplane and rc car with carburated engines, they have needles inside the carburator

the point is they all requre adjustment, the webber might have a screw or something for adjustent.

i think the problem is if u havent adjusted anything (like my relative:headbang:), since carburator car = NON electronic, u have to actually manualy adjust it.

i think ur carburator is running way too rich when the throttle is opened, there are lots of ways to adjust it, some people turn the needle screw a certain ammount, what u can do is simply look at the color of the EXHAUST comming out, it has to look clean, I would even adjust the mixture to LEAN, most vehicles run BETTER and have more power when running LEAN (Less gas more air)

I hope that helps, i would google adjusting ur carburator. remember its completly mechanical

i wouldnt worry about ur missing thing, it does not look imporant :lol:
 

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Tinkering Addict
89 Tercel EZ
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332 Posts
Discussion Starter · #6 · (Edited)
Im not realy an expert at all so...

well my relative has a MG car its carburated too, and i used to have radio controlled airplane and rc car with carburated engines, they have needles inside the carburator

the point is they all requre adjustment, the webber might have a screw or something for adjustent.

i think the problem is if u havent adjusted anything (like my relative:headbang:), since carburator car = NON electronic, u have to actually manualy adjust it.

i think ur carburator is running way too rich when the throttle is opened, there are lots of ways to adjust it, some people turn the needle screw a certain ammount, what u can do is simply look at the color of the EXHAUST comming out, it has to look clean, I would even adjust the mixture to LEAN, most vehicles run BETTER and have more power when running LEAN (Less gas more air)

I hope that helps, i would google adjusting ur carburator. remember its completly mechanical

i wouldnt worry about ur missing thing, it does not look imporant :lol:
Thanks for the response. A guy at my work who used to fix cars also mentioned the needle maybe having some particle stuck in or around it or something like that. I'll have to look at that I guess. I really don't know anything about working on the insides of carburetors, though the Weber looks simple enough. All my problems w/ carb'd cars have been emissions equipment related until this tercel. The fuel/air mix is very close to if not perfect and I have tweaked with it with no apparent effect on the problem. Exhaust is normal color except when it spazzes out.

It happens most when it rains or when it is really humid and hot outside. The problem is getting worse and is now happening at slower speeds driving around town. It behaves like I would expect a car with bad plugs and/or wires to do, but that stuff is all new and has been replaced twice just to be sure. I also replaced the fuel filter (again) just to make sure it wasn't clogged.

The car acts it's trying to headbang (as in:banger:) which makes me :headbang:. It's bad enough that I have hit my head on the steering wheel because of it. Does anyone know if there is anything else besides plugs & wires that could be affected by the humidity/rain in this way? Is there some other critical electric connection that could be affected maybe?

EDIT:

I found out what widget belongs in the place in the picture above while looking through some old pictures of my engine compartment. It turns the vacuum from the manifold into a steady source of slight vacuum - like a pressure regulator. Attached it and it seems to have no obvious effect on the car's behavior... probably serves to keep high vacuum from damaging the vacuum sensor or something like that.

That got me thinking that maybe I need to open the fuel pressure regulator a little more to increase fuel flow under load. Doing so made enough of a difference that I had to readjust my fuel/air mixture afterwards... car was misbehaving right before that adjustment and worked fine when I drove off afterward. Crossing fingers (again) that it's fixed, though it still doesn't explain why humidity matters...
 

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Tinkering Addict
89 Tercel EZ
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Discussion Starter · #7 · (Edited)
Well the problem remains... I've noticed a new symptom that may be related...

The idle sometimes gets stuck at around 1100rpm when I let off the gas and will drop to the usual 800rpm when I gently tap the throttle a few times. The throttle is fully releasing down to the idle screw on the carb though. I'm thinking it may be the vacuum advance module not releasing properly, perhaps getting stuck as it's releasing. I have already checked the timing with a light and it is correct without the vacuum advance hoses connected. The throttle drops as it should with the vacuum advance disconnected, so I think it is causing this. The timing marks next to the crank pulley only has a range that goes to 10*BTDC, so it's hard to tell what's happening beyond that.

Anyone know of a good way to test the vacuum advance module to make sure it's working properly and smoothly? Is it possible to clean or lube it?

EDIT: Also, my speedometer needle wobbles when accelerating but the tachometer doesn't, which it did not do before... dunno if that's related

Any insight is appreciated

EDIT2: Sprayed some WD-40 into the vacuum advance hoses and let it run into the advance unit per another forum's suggestion. The bucking/stuttering immediately got softer and less severe, but didn't go away completely. Ordered replacement distributor w/ advance.
 

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Replacing the distributor with one from another car seems to have solved the problem. It's been about 3 weeks since I replaced it and haven't had any problems since.
What do you mean another car? Another Tercel or something completely different? I too have the same problem. I've been running my stock carb richer, which helps, but doesn't solve the problem.
 

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Tinkering Addict
89 Tercel EZ
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332 Posts
Discussion Starter · #10 ·
I replaced with a distributor off another 89 Tercel. My fuel pump failed not long after, so that may have been part of my problem too. In retrospect, I probably had a dual problem of low fuel flow from failing pump and failing distributor. Keep in mind that in my case this particular issue started after I already had replaced the stock carb, which can (and often does) cause these problems all by itself.

In your case I'd first check for vacuum leaks and faulty pollution control widgets, especially any with three hoses going in - they are vacuum switches with rubber inside that degrades over time and eventually ruptures causing vacuum leaks that lean out the mixture and screw up vacuum flow to other pollution control widgets. You can also test ignition coil resistance and fuel pump flow before replacing.
 

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I was going to say check your ignition timing with a timing light but your problem went away ! You should check your ignition timing anyway !

Remove the vacuum hose from your distributor and check for timing : 3 degrees BTDC
Replug the vacuum hose from your distributor and check for timing : 12-18 degrees BTDC

The steps must be done when the engine is hot and idling at maximum 950 rpm.
 
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