Engine dies after rough idle when stopping

[cromagmom]

I was hoping fffutr's recent post would be able to help me, but that was a no go for me.

Symptoms: Warm engine idles very rough around 400-600 when I'm slowing down, cornering, stopping. Been having to reignite mid-turn or at stop lights, and that gets everything going again.

Just reseated all of the vac hoses, couldn't find any leaks with carb cleaner, checked all the idles, seem to be OK.

Maybe unrelated, maybe not:
-T4 seems to take extra long to warmup before I can drive, even on warmish days. A good few minutes before the thermometer budges.
-Alternator belt getting worn. I can see some belt threads. Belt tension seems fine.

[Mark]

Still seems like a vacuum leak. If the hoses are fine, rule out the diaphragms one by one by unplugging/capping the hoses to the various diaphragms and see if the idle improves. My bet is on the EGR vacuum modulator if it only happens when warm. At least that's what it was on mine with the same symptoms.

[dlb]

agreed on the vacuum leak. check all diaphragms and make sure the hoses are routed correctly. also check that the big hose that goes to the underside of the air filter housing is connected. it is a little tricky to connect when reinstalling the housing and is a massive vacuum leak if forgotten.

but i would also check the choke valve and make sure it's wide open once warm, check the AAP for leaking, and check the carb fuel level using the sight glass on the front of the carb, check the throttle positioner setting.

[Petros]

mine acted like this without any vac leaks, turns out it was the throttle positioner. this is the little vac operated device on the front side of the throttle shaft, it is designed to stop the throttle plate from slamming shut (when that happens it causes a vacuum spike that sucks in too much fuel, so it reduces emissions and actually improves fuel economy slightly). it also improves drivablity of this lean running carb.

You can inspect the operation by visually observing it, you rev the engine and than allow the throttle to drop to the closed position, the little device should catch the throttle shaft at the end of its travel and than close it slower over the last 5 to 10 degrees of throttle position to the idle position. For some reason the adjustment was backed off on mine so it did not contact the bell crank on the shaft, and every time I came to a stop the idle speed would drop way down, sometimes stalling, and than returning back to normal after a few seconds. by adjusting it just enough to stop this behavior it improved the sudden engine slow down and rough idle issue.

If there is no vac line connected, or the diaphragm is damaged, it will not work properly. usually it just needs to be adjusted, but on a car this old anything is possible. Also, verify the vac line on it is connected to the correct port from the vac diagram in the service manual.

[cromagmom]

Thanks, y'all. The throttle positioner my have been the culprit, and also setting idle according to the FSM. I just set the idle as I was driving around and erred on the high side so that it would stop cutting out on me. Two of the vac tubes were switched on the TVSV, but I am not sure if that made much of a difference for the symptoms I was having.

At 150k, would cleaning the EGR be a useful exercise?

[ARCHINSTL]

At 150k, would cleaning the EGR be a useful exercise?

YES! Mine was blocked solid with carbon.
See viewtopic.php?f=4&t=906&hilit=egr
See viewtopic.php?f=7&t=1496&hilit=egr for more pix
Don't forget to clean the big "source tube" from the manifold to the EGR valve; it is usually loaded with carbon as well.
Do a search here for "EGR modulator" - be sure to use the quotation marks! Also review the FSM's section on it.

Of course I no longer need it, as I installed a Weber in 2008 and blocked off both the EGR port and the "source tube's" manifold port.
Tom M.

[irowiki]

Generally some have just removed the EGR altogether with good results. My 81 had it removed and ran fine, and it is blocked off on my old 84, and it runs fine too.

[dlb]

i disabled the EGR on my corolla while troubleshooting what i thought was a vacuum leak, and noticed predetonation once the engine was warmed up. i had previously advanced the timing 5* beyond stock so that had a lot to do with it but i've always read that EGRs lower combustion chamber temps, and this experience certainly proved it to me.

[cromagmom]

Well, though I had solved my problem, but perhaps just made new ones. Everything is fine when the engine is warm, now the issue occurs when cold, which was probably happening all along.

Looks like I just need to carefully go through the FSM from the start and get everything keyed in.

Think I'll leave the EGR alone for now, but I like that use of old bike parts for cleaning it up, archinstl.

[ARCHINSTL]

Think I'll leave the EGR alone for now, but I like that use of old bike parts for cleaning it up, archinstl.

Thread heist! Thread heist!
Interesting - I owned a shop here in STL from '73 until '03! Are you in the trade?
Tom M.

[cromagmom]

Sorta...I used to run a bicycle co op in NC. We refurbished and loaned bikes out for people interested in commuting who couldn't afford a new bike. Not being the greatest capitalists, we struggled for a bit after losing our free shop space, and then it went under when I moved away. Other than that, I'm a big fan of touring, though I haven't done anything too extended since a trans am tour a few years ago.

[cromagmom]

Found out my plastic jet coming out of the choke breaker was clogged. Also, I found it that it is very brittle. I'm going to just cut some new vac line for now, but is there anywhere to pick up a new one?

[irowiki]

Nah, it isn't clogged, it's supposed to be super restricted!

[Mark]

The choke breaker is just supposed to be a restricted orfice to delay the vacuum building up in the choke breaker diaphragm by about a second. A cheap replacement that may or may not work is to pinch the vacuum hose with a clothes pin to narrow it a bit. To be honest, you could probably just do without it and just run the vacuum line straight to the choke breaker. Unless it's very cold, I don't think the delay when the choke is fully closed has much of an effect. I'd just let the choke breaker crack the choke slightly open right away.

[cromagmom]

I'm going with the line straight to the CB. The FSM has a picture that leads me to believe there should be some amount of sensible air flow when you blow on it from either end. Now that it is off, 1.5 problems seem to have been fixed! I can drive away after only a few seconds of warming up (this wasn't even the case in warmer months), and there is no stalling at warm or really cold engine temps. On the way to warming up, though, it still stalls at stops. Just a little bit more fiddling I suppose.