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Old 10-02-2012, 11:23 PM   #1 (permalink)
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Carb rough idle

EDIT: Solved.

This is an old-school carburetted Toyota 4A-F engine (1989). I realize I may just be griping about the nature of the beast. Fuel injection is so much more consistent. Althought it is carburetted, it does have some intermediate emissions tech on it, O2 sensor and presumably stepper motor, 3-way cat.

But I doubt it ran like this when it was new. Maybe there's something worn I can fix or adjust. If you think I should let it go just say so.

It idles rough. After setting the idle speed (supposed to be 650) it wanders between 550 and 750. I had to adjust the idle speed set screw after it started stalling at idle a few days ago, that got me into tune up mode. The cause may just be that I reduced the octane from 89 to 85 on the last tank, and now it wants to be tuned for low octane. But regardless, now I'm in tune-up-mode, and it has been idling rough for a long time. Some other misbehaviors, or maybe just old carb behaviors: 1) it may hesitate if I try to accelerate at low RPM while cold; 2) after sitting in the driveway a week, I have to pump the accelerator 5-7 times before it will start, sometimes I have to pump the accelerator a couple times after it sits overnight.

I checked the timing with my timing light. Actually I'm not sure if I changed the timing because I moved the distributor and then remembered to remove and plug distributor vacuum lines. So the timing is now correct, whether it was right before or not. I cleaned the indicator plate for the first time, so for all I know I misred it when I tuned it ages ago.

Compression on this engine was perfect when I checked it 13k ago, although I feel like it runs a little smoother when I use high-mileage engine oil.

I had the mechanic clean and tune the carb about 3k ago, didn't make much difference that I noticed. He did say I should rebuild the carb.

Ideas?
  • Could it be a bad / poor O2 sensor?
  • Could it be valve clearance? When I checked that 13k ago, they were in spec, but not consistent one with another. 5-8 mil range on intake. 8-10 mil range on exhaust. Should I go back in and adjust them so they are all consistent?
  • Can I rule out a vacuum leak? The brake boost is alive before I start the engine after a weekend of resting. I got a "stethascope" from a length of 1/4" hose, plus ear muffs, and couldn't hear any hissing anywhere in the engine compartment, except what I think is air going through the throttle (I can hear it with the stethascope off coming from the air filter vicinity, but can't find any locality for it with the stethascope).
  • Time to listen to my mechanic, rebuild the carb?
  • I've been over and over and over the distributor cap, rotor, wires, and spark plugs. Pretty sure I've replaced all of them inside of the last 15k, not with any noticeable change. Spark gaps are all 43 mil, as speced, and they all seem to have about the same coloration. However, if I (carfully) unplug the spark wires one at a time, I notice that cylinders 1 and 2 strongly affect idle speed, but the effect is quite subtle for cylinders 3 and 4.


Last edited by christofoo; 12-11-2012 at 03:05 PM..
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Old 10-02-2012, 11:50 PM   #2 (permalink)
ron
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get a pertronics electronic distributor kit and get rid of the points if it is equiped with it now. buy a rebuild kit for the carb, and aviation form-a-gasket for the edges of the gaskets. but before that rule out a vacuum leak with a squirt gun , worm and idle the engine,squirt the vac. line connections if you have a leak the engine should smooth for a moment. good luck
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Old 10-03-2012, 12:05 AM   #3 (permalink)
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Sounds like its idling lean (both from the pumping and especially stumbling upon acceleration.)

As a diagnostic test, try idling and revving it with the choke manually applied. Have an assistant block off 75% of inlet, either by choke valve or even fingers! If it smooths out, you have your culprit.

Also pull the plugs and read em.

If it turns out it idles lean, you need to find out why. Blocked jet, leaking gasket are possibilities. Also the Byzantine feedback mechanisms.

P.S. Is your Carb a Constant-velocity type? If so, a leaking CV diaphragm is quite possible.

Good luck!
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Old 10-03-2012, 01:15 PM   #4 (permalink)
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Here are the spark plugs. I lied, they are not all the same. They are ordered 1-4 from left to right. 1 and 2 are the ones I said clearly affected the idle speed, 3 and 4 are the ones I said have only a subtle effect on idle speed.



I would swear I've seen the black deposit on plug 1 come and go.

What do these plugs mean?

I've changed all the electrical components at least once, although I never saw any visible problems. Does this photo and my observations of idle speed result in favoring an electronic distributor upgrade? Yes, the current distributor has points. EDIT: no it doesn't. The distributor is mechanical, the points are electrical.
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Last edited by christofoo; 10-29-2012 at 04:35 PM..
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Old 10-03-2012, 02:52 PM   #5 (permalink)
ron
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1 has rich or oil , 2 looks normal, 3,4 normal or no spark,or lean condition at idle. sounds crazy but what do they smell like,oil,fuel. as I recall it runs ok at speed ,how is your charging system & battery may not be enough power output till its up in rpm's or fuel pressure is low at idle . Lots for you to check .
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Old 10-03-2012, 03:08 PM   #6 (permalink)
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Quote:
Originally Posted by ron View Post
1 has rich or oil , 2 looks normal, 3,4 normal or no spark,or lean condition at idle. sounds crazy but what do they smell like,oil,fuel. as I recall it runs ok at speed ,how is your charging system & battery may not be enough power output till its up in rpm's or fuel pressure is low at idle . Lots for you to check .
I remember sniffing plug 1 once and not detecting anything in particular, maybe a little fuel... but you know when I switch the key off the it's the ignition that dies, the manifold stays wet and the cylinders will end up with a little fuel sitting in them (right?). That's why this type of engine can diesel after killing the key if the plugs are too hot (timing over-advanced), similarly sometimes if I don't pump the pedal before starting it will start right away but then stall.

At speed it usually runs okay, a while back I was having a little shudder in gear at low-to-mid throttle that I thought might be misfiring.

I do have a volt meter in the accessory port and I cleaned up the battery with a desulfating charger, so I do know the battery / charging system is in good shape.

Last night tinkering with things I noticed that once in a few hours I will hear a tiny little bubbling sound (high frequency, like a buzz) for about 30 seconds. Next time I will have my stethoscope ready and be able to pinpoint it. I'm betting that air is getting drawn into a fuel line somewhere, and the fuel is backing down the line, and fixing it will eliminate the need to pump it before starting. I doubt that air in the lines is the explanation for rough idle though; it idles rough even after a 10 mile drive. (Unless the air entry is occurring much faster when the engine is turning.)

Last edited by christofoo; 10-03-2012 at 03:17 PM..
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Old 10-03-2012, 03:36 PM   #7 (permalink)
ron
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if you have a leak on the suction side of the pump it should be worse at speed with higher fuel need, not to mention that the fuel should show up as a puddle or strong fuel smell when not running. I think the real issue is you need someone to properly trouble shoot this thing with hands on,Like the mechanic who said rebuild the carb . do it yourself if your are able and save $ . it sounds like a place to start
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Old 11-05-2012, 01:49 PM   #8 (permalink)
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Quote:
Originally Posted by christofoo View Post
Here are the spark plugs. I lied, they are not all the same. They are ordered 1-4 from left to right. 1 and 2 are the ones I said clearly affected the idle speed, 3 and 4 are the ones I said have only a subtle effect on idle speed.
...
After some (more) googling, I found this:
89 corolla idles poorly. - Toyota Nation Forum : Toyota Car and Truck Forums

After running for a few seconds, I pulled off the vacuum line from the AAP to the thermal vacuum switch and found gas in it, almost certainly confirming that I have a fault in the AAP diaphragm leading to flooding of cylinder 1 via the vacuum lines, as described in that post. The presence of gas in that line, combined with the color of the cylinder 1 plug, together seem like smoking gun evidence that the AAP diaphragm was at fault. If there is any alternative explanation, I wouldn't know what it is.

EDIT: I never did have any backfiring, such as other users reported. It may be that my AAP diaphragm rupture had not progressed the point that it could cause backfiring.

So I have disconnected and plugged that vacuum line (eliminating the flooding at the expense of killing the AAP, not that it was probably working before), and although a single 10 mi drive is far from definitive given the subjective nature of the symptoms, I notice: less "misfiring", smoother idle, quicker kill with less tumbling for EOC.

I'm excited about what this might mean for mileage and emissions. I'm due for a tailpipe test in Jan (required in my area). It'll be interesting to see if there is a significant difference.

HOWEVER: I've become suspicious and still am suspicious of my EGR and will be doing some checks on the valve, maybe even open it up and look at the deposit levels, later this week. The one symptom that I think the AAP diaphragm fault doesn't explain is that (I think) I have some pre-detonation when using lower octane gas. Also, with the engine at 223k, I think it's worth a look.

Last edited by christofoo; 11-05-2012 at 01:57 PM..
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Old 11-11-2012, 01:45 AM   #9 (permalink)
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EGR is fine

It looks like I'm seeing an MPG improvement after the AAP vacuum line plug, especially for this time of year. I still feel the engine is subjectively smoother. Later on I'm going to reread the plugs, too.

In other news, I ran an EGR test today. It's working fine. Test method: using a hand vacuum pump with gauge to verify a) the EGR valve diaphragm holds a vacuum, b) the engine dies if the EGR is opened manually during idle. Separately, I verified that the EGR diaphragm moves when the engine is reved. That about covers it?

So apparently my hunch was wrong, no EGR fault.

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