Slow idle when engine hot

justinshelton · Post by **justinshelton** » Wed Jun 09, 2004 7:37 am

Me again, you all must think my car's a piece of crap given how often I post here. But it's really not that bad.

Anyway, my newest issue (actually, probably not new, but connected with the advent of hot weather, so my engine is running hotter) is that when I start my car and poke around town, the idle is fine when I come to a stop light and have my foot on the break with the car in gear (we're talking automatic tranny.) But if I drive on the highway and then come up to a stop with foot on break and car in gear, the idle seems way low, like it might just die. A few times I've shifted into neutral, and the idle seems fine. Then I shift into gear when my light changes to green.

How do I fix this???? The same thing happened to one of those awful auto-stick shift bugs I had back in high school, but since is was a major POS and I didn't really feel mechanically empowered, I just always shifted into neutral at red lights. But I know now that I do not have to live my life with these little irritants.

Car has recently been tuned, and tonight I'm going to replace the fuel line and put on a Pertronix. I'll give other things a go while I'm at it to sort out the iffy-idle-when-hot issue.

Thanks again!

- Justin

minty73 · Post by **minty73** » Wed Jun 09, 2004 9:22 am

i have the same problem on my manual, when its hot and i brake to a stop it stalls but if i coast to a stop it doesnt do it! weird, tried the timing thats fine, all new ignition system too.

find it strange that it only happens when i am using the brake!

DeathBus · Post by **DeathBus** » Wed Jun 09, 2004 10:35 am

Vacume Lines? Wouldnt hurt to check them all.

raygreenwood · Post by **raygreenwood** » Wed Jun 09, 2004 10:50 am

Its a temperature related fuel mixture issue. First off, on the automatic, make sure that gold dashpot with vacuum line near the TB is adjusted prroperly and operating. Next, check the cold and hot readings of your CHT. It may be out of spec. Next...if you have the intake air temp snsor still connected...disconnect it. Generally idle will pop right up. You may need to make a fuel mixture tweek after that. The intake air temp sensor increase fuel mixture from between 5 and 10% acrossthe board. It does not do well in hot climates. It banks heat too easily in the hot plenum. Don't worry about it...its not a real necessity. I plug it back in in cold weather.
The CHT is usually the real culprit here. Its getting down into the lower end max range due to ambient. I think Brad Anders had noted that almost all of the effect of the range of the CHT is in by around 225F or so, but...I know from living in hot climates, i get best running by adding about 125-200 ohms of ballast reistance to the CHT line so It never drops to the bottom range.

Check all of your plugs and connectors for cleanliness. Lastly...and this is common, check your fuel pressure when the wetaher is hot. the fuel pumps...especially old ones....can drop a few PSI when they get hot. Ray

justinshelton · Post by **justinshelton** » Wed Jun 09, 2004 2:57 pm

Thanks yet again, Ray. I will work through yoru list of suggestions. But one question: what is the CHT? And the TB?

- justin

raygreenwood · Post by **raygreenwood** » Wed Jun 09, 2004 6:28 pm

CHT is cylinder head temp sensor....its on the 3/4 head...single wire. The TB is the throttle body or butterfly plate. There is a vacuum fed dashpot on the automatics, that slows the throttle closing just slightly...when you drop off the pedal and come to a stop. It keeps the idel from dropping to low on an automatic transmisson car, when coming to a stop at the light. Ray

ploug · Post by **ploug** » Sat Aug 08, 2009 8:13 am

i have a similar problem.

d-jet and auto tranny.

only when hot.

it idles good in neutral but when i put it in drive, with the brakes on it almost stalls
RPM around 5-600 oil lamp flickering or on.

i've adjustet the fuel pressure up to 30 psi (it was at around 22 - guage schould be accurate)

the car drives better then ever besides this problem.

could it be that i now is runnig too rich at idle ? - before the pressure adjustment i had a co meassurement of 4.3 % at idle.

i have also tried to pull of the air intake sensor plug - but engine ran worse and almost died. - pluged it in again and idle at neutral rose to around 900.

does the gold dashpot have anything to do with this ? i how do i adjust it ?

raygreenwood · Post by **raygreenwood** » Sat Aug 08, 2009 11:59 am

Yes, on the automatic, the gold daspot is designed to "blip" the throttle a bit as teh transmission finishes disengaging...so to speak. The symptom is that when you come to a complete stop at the light teh car either stalls or very nearly stalls. A quick rev by your right foot usulaly sets it right even though idle may be low.

The dashpot is adjusted by reving to a specific rpm...I'll have to check the book....but its low....like 1100 to 1200 and then releasing the throttle. When you release the throttle you should see the plunger immediately move forward and contact the lever on the throttle ....just enough to keep idle stable....not rev the engine......and slowly allow the throttle to close over about 1.5 to 2.0 seconds.

Its common that the diaphram may have a vacuum leak. Test that too.
There are two adjustments if memory serves. One at the bracket that holds the diaphram and that I think adjusts the speed or vacuum sensitivity....the other is the pushrod adjustment with locknut.

Also check your CHT ohm reading when the car is warmed up. In hot weather, a reading of 100 or lower is too low. You can either add a resistor inline to it of 100-150 ohms or make a stand-off spacer for it teh CHT. Ray

ploug · Post by **ploug** » Sat Aug 15, 2009 8:23 am

ok

the dashpot seems fine. and the CHT reads 150 ohm when hot.
i also changed the points for a pertronix ignitor.

so far so good...

i still have the problem. comming to a stoplight it almost dies, but now the RPM goes up and down up and down... between 700-500 RPM - if i leave it in gear it will after it's little dance stall.

same thing happens if i change from N to D with the foot on the brake.

in N it idles steadely around 1000 RPM.

could the vacuum modulator valve on the transmission have anything to do with this ?

fuel or ignition problem perhaps ?

i'm pretty stomped, so i hope you guy's have some ideas

raygreenwood · Post by **raygreenwood** » Sun Aug 16, 2009 1:52 pm

Oh for sure there are fuel variances. I have not found a D-jet injected car yet....that did not need slight tweeks to teh MPS. Bear in mind....there is veirtually no way that any car after 30+ years has the same vacuum signature it left the factory with....and the MPS was set for that.
But...before fiddling with the MPS and your fuel mixture.

First....you probably need to adjust the gold dashpot. Its job is to do exactly what you are sing. When you come to a stop.....there is load on the engine but vacuum is high because the throttle is closed....and therefore enrichment to keep idle up until the pressure
load has bled from the torque converter....is not there. So....at highest vacuum....throttle closed....it pops open to push your throttle open a slight amount. What you usually see is this: Put the car in gear, set the parking brake. Rev the engine only to high idle..maybe 1200...just a blip of the throttle. Release it.
In an instant as the rpms drops the Gold dashpot will pop out and prod the throttle. The rpm will go up...maybe 200 rpm. I f its working properly...it shoudl settle down at that point to a normal idle. If its not working properly...it will idle down low and the dashpot will pop out again...prodding the throttle. It can do this several times.

The object is that when vacuum goes to maximum setting...and the tranny shifts down to 1...the vacuum breaker...which is the deceleration valve...pops open to bleed off vacuum...causing the gold dashpot to pop open.

You need to check the electrical deceleration valve and its switch that is one teh automatic transmission. There are several compoents...a relay and logic circuit, the vacuum breaker and the tranny switch. They are all on the left side of the engine compartment.

Also...get a decent vacuum gauge. Its quite possible that the deceleration valve might be causing problems. And yes....it could be the vacuum modulator....but only if its leaking vacuum. Thats another of those parts that in my opinion should be adjusted properly as per the automatic section in the brown type 3 bentley.
Because....it governs shift pressure. Also...because.....I believe that the decel valve switch is a hydraulic pressuer switch. So...if your main hydraulic pressure in the auto tranny (governed by the modulator valve) is low or borderline.....it will screw with the deceleration valve.

I have gotten around this...by installing a manual pneumatic style decel valve from a manual transmission car....and adjusting it open at max vacuum...idle. It works...but is hard to adjust and will screw with your gas milage slightly. Ray

ploug · Post by **ploug** » Mon Aug 17, 2009 11:55 pm

ok

since i do not have a deceleration valve - nothing to check there.

plan is to check:

timing
valves
golddashpot
triggerpoints
fuelpressure regulator

perhaps make a fixed oriface for the PCV ?

Post by **david58** » Tue Aug 18, 2009 4:44 am

ploug wrote:ok

since i do not have a deceleration valve - nothing to check there.

Well it sounds like you found the problem then.

raygreenwood · Post by **raygreenwood** » Tue Aug 18, 2009 5:12 am

Thats an interesting point. I know the deceleration valve was "technically" emmissions equipment.
Now...on US cars....the dashpot is adjusted to work in concert with the deceleration valve. I don't know if other cars may not have had it.

The odd thing is that you see literature claiming that automatic transaxle cars had an electric decel valve. I have found that to be not entirely true. Looking back in my old notebooks....my first 411 sedan, my blue 411 wagon and my gold 412 4 door all came with automatics and all had factory manual decel valves (vacuum operated).

The gold dashpot I have found to always be a problem. As they age they get out of spec.
150 ohms is not especially bad or low for CHT reading....but...what is your external ambient temperature?

Also, what is your timing set at?
Before proceeding too much further....lets look at the basics,
What is your fuel pressure.
Also....it may be a good idea to have the bands adjusted...or adjust them yourself and then do the testing to adjust main pressure at the modulator valve on the transmission.
This condition is partly vacuum related and partly transmission pressure related.
But....there area great many things that can affect vacuum on this car. Ray

ploug · Post by **ploug** » Thu Aug 20, 2009 1:45 pm

great news

it turned out to be a loose/bad connection at the triggerpoints.
it now drives rock steady.

is there a way to thigten the female connecters ?

what is the best cleaning agent for the connecters ?

raygreenwood · Post by **raygreenwood** » Thu Aug 20, 2009 1:56 pm

Fast evaporating electrical contact cleaner works well.

Variable idle masquerading as a vacuum leak.....yep...that could mess with the gold dashpot...and evreything else.

Those loosening female connectors are why I hate teh D-jet harnesses...and why no one has ever used that connector on another EFI system again.
You will have to remove them from the plug with a small flat pick or a jewelers screwdriver. Look at the openings where teh make tabds stick into the plug and you will see an indention where you slide in a flat pick to push in the brass locking tab on the connectors. You then pull teh connector out and lightly crimp the rolled edges with pliers to make them tight. A smarter thing is simply to clip them off and crimp on new ones. An even smarter thing to do is graft on a threewire modified plug out of a water cooled audi or VW that uses modern terminals. Ray