Hi. I know that the engines with manual transmissions have a 'device' that holds the RPM's up when you clutch between gears. I think mine holds too high for too long, and basically I think I'm wearing the clutch out faster when the transmission slows the engine down when I get into the next gear. Where is the throttle 'dashpot' device (if any), and how can I repair/replace/test/disable it???

Secondly, I am planning to replace the engine (with only 60k miles on it, and is now starting to smoke a little when I start it) in a few years, and I still have my original clutch. If I replace the clutch now, will I be able to re-use it with the new small block? Or should I just wait, and change it when I get the new engine?

Thanks! -Brian

 

There's a device like that? No kidding! I'd like to hear where it is, too... I have an automatic car, just curious...

If you're replacing the 2.8 with a v8, then, no, the clutch won't work... even if it did work, with the v6 transmission, the v8 will beat the life out of that old clutch and old trans! The axle and driveshaft are the same exact strength as those on v8 cars, so those will be okay.

 

I was under the impression that my Borg-Warner 5-spd would handle the torque of the V8... Hopefully I will drop in either a Pontiac 3rd gen Formula 350 engine, or a 350 long block crate engine. But first, I need to pay off my Grand Prix.

From what I think I know, I just need to swap the engine, mounts, front springs, bell housing, ecm and swap a couple wires. I have already purchased the V8 gauge package for the dash, and it's ready to drop in.

 

It'll hold a 305 maybe... a 350, you'd better hope everything is in the correct working order, otherwise your screwed...

Not naysaying... just saying it there is an if in you 3.1's trans, the 350 will rip it a new a-hole...

 

TTT

Anyone know about the 'device' that holds the RPMs up?

 

Yeah, its called your foot. I've never heard of such device, nor have I seen a engine hold it's RPM's with the clutch applied.

 

Well, with the clutch on the floor, and the tranny in neutral, I can tap the gas, and whatever peak rpm it hits, it will stay there for 1-2 seconds before descending to idle speed. Figure that one out...

 

um... it's pulling the air from the plenum (behind the throttle plate), and running on that, until vacuum is increased - bringing down the revs?

 

So, why do my other 2 cars not do this? One is a '94 3100 (auto) and the other is a '97 3800II (auto). When I youch the gas in neutral on those two, the rpms come back down fast. Sometimes on my '97, it almost stalls out. (Because my IAC motor does not catch up fast enough)

 

Where's that "worst thread ever" pic when you need it?

So are you insisting that this engine has the magical device that other cars don't??? I seriously don't understand what you are trying to figure out.

Lets see...on your car there should be one cable on the throttle- the accelerator cable. It's connected to nothing but the gas pedal so we can rule that out RIGHT???

Then there is the PCM. It can open the IAC and increase injector pulse width......except that the IAC can only support the motor to about 2500 RPM....so we can rule that out RIGHT?????

What else is there??? NOTHING.

And Doward is right, it will be able to run off of the air still in the plenum for a short time.

 

A stick will do that, just as I said. Why? When you hit the clutch, you totally seperate the engine and transmission. The engine has NO LOAD ON IT, so it needs very little air/fuel to run at a certain rpm.

And automatic ALWAYS places a load on the engine. You toss it to neutral, and all you've done is take out the gearing. The engine is STILL turning the transmission gear set, and the viscous Tranny Fluid in the torque converter. The planetary gears in the auto are not meshed together, so you don't rotate the driveshaft, but you STILL TURN THE GEARS!

That parasitic drag of the fluid in the torque converter is why a stick is faster than an automatic 90% of the time - all the power goes to the wheels, and none is used forcing the fluid to turn.

****, there goes my not knowing anything disguise.

 

You gave that disguise away when you said "parasitic drag"

 

Well, hey, I'll through one more thing in here -

"Well, why are all the fastest cars automatics?"

Because the TH400 is almost indestructible (I think a nuke could take it out... IF it hit it right - LOL) and a well set up automatic can handle more power than a stick.

Take two engines, say... 300hp at the flywheel, all factors the same, just one is a stick, the other an auto, and the stick will always put down more power to the rear wheels - UNLESS you've got a lockup converter, and lockup as you run the dyno.

Too many factors, all things considered, a stick is faster.

Besides, it's been said -

When you use a stick, you drive the car.
When you use an auto, you steer the car.

 

Ok, (trying to keep this thread friendly...) remember that the carb'd engines have an oil dashpot that holds the throttle blade open for a second when you quickly take your foot off the gas? This stops the engine from stalling out.

I've been told by GM part's guys (keeping in mind that they may be idiots...) that there is an electronic version of the old mechanical dashpot, for the injected vehicles.




By the way, some of the good "Worst thread ever" pics can be found in the AAG forum over to ClubGP.com, where I reign as the local electronics guru... *grin*

Well, If you'd like to drive to VT, I can SHOW you one that does!

 

its called a PROM inside the ECM

seriously, manual PROMs are calibrated differently so the RPM's wont fall as fast when you clutch

it may not HOLD the RPMs per say, but it does not allow them to fall as fast

 

As I sit and think about this some more... I recall a few years ago that I threw an EGR code... I cleaned the EGR, replaced the gasket, and threw it back in. After resetting the ECM, the code went away, and I have never seen it since.

It is possible that my EGR valve (or the control solenoid) is sticking open, and causing my RPM's to hold up, and give me a rough idle occasionally?

 

Ok, I'm curious as to just what carb that dashpot is on? Cause there are two dashpots on all QJets I've seen -

The one you've got circled would be the choke dashpot. Holds the choke flap closed (creating enough vacuum to pull the gas through the main/idle circuits creating a VERY rich A/F to start the car) until the engine starts up.

The second dashpot controls the secondary air valve opening rate. Prevents a lean "bog" from the secondaries coming online before the secondary jets can meter fuel into the new air coming in.

I believe what you're refering to, is the 'fast idle cam' that will prevent the primaries from closing completely before the engine has completely reached operating temps.



I'm still sticking to my theory of the excess plenum air feeding the engine until vacuum is created, slowing the rpms back down.

But hey, feel free to prove me wrong!

 

just a little more FYI:

On TBI setups, that dashpot would control choke/fast idle settings, as previously described.

A stuck EGR can cause rough idle, but not hold rpms. a stuck EGR at idle will actually try to stall your car, lowering the idle rpm.

I'd check your wires/plugs first, that was the cause of my rough idle.

Then I'd check the IAC valve...

 

I have new: cap, rotor, wires, coil, plugs, fuel filter & TPS. Cleaned IAC valve seat and pintle over the weekend. I usually clean it every year anyway. Any additional suggestions?

 

If you are SURE the wires/plugs are good, I'd run a compression test next, just to be sure that I had good compression all the way around. Nothing quite like a cylinder running low compression, to make the idle rough...

 

Just reread your first post - what color is the smoke on startup?

 

Guys, The only reason why manuals take longer to wind down is the scientific term "Inertia". Flywheels on manuals are heavier than flexplates on automatics and take longer to wind up and wind back down.

edit

That is unless you have a low stall converter. High stalls will drop faster

 

Doward: The smoke is a blueish-white. I believe that it is oil seeping around my valve guide seals. No big deal, I just started using the Valvoline high-mileage motor oil. I change the oil once a year, wether it needs it or not, which is about at 700 mile intervals. Yes, I drive the car a mere 700-800 miles / year. (And only with the tops off!)

AF: The idle plateaus at a high RPM, hangs there for 2-3 seconds, then drops down fairly quickly to idle. It's not a smooth deceleration curve, it really 'hangs' at 2k to 3k rpm, then drops off. It's really nothing I've seen before in an automobile engine.

 

That sound to me like there is something screwy going on with your computer/sensors. My suggestion is to have it hooked up to a diagnotic scanner to see exactly what the vitals are reading. You must be getting fuel pulses from a faulty reading in your electronics.

 

I have an 86 that was auto and now is stick and it does the same thing. Who cares.
My car has next to no oil pressure and smokes when you start it but it revs and keeps the pressure up. The only time I notice where the revs stay high is when I beat on it and come to a red light and take it outta gear and coast to the light. Some one already said you open the throttle so much that it's sucked back so much air that it's almost compacted in there and needs to burn off.
Also if you take your motor to 5500 there's alot of momentum that you've created. If your motor loses that momentum quickly, like something is binding, you are going to have serious problems.
The bottom line is WHO CARES. It runs doesn't it. If anyone asks, tell them its a speed secret.

 

"WHO CARES?!?!?" Well, for starters... I DO. I happen to like this car...

The RPM's stay up too long between shifts, and the clutch is pulling the RPM's down to match. not only is this extra wear on my clutch, it makes driving a pain in the ***, having to hold the clutch down for 3 seconds between gears. My oil pressure is normal, and I just changed my O2 sensor.

You find me another 17-year old Vermont car that looks like this:






I am it's second owner. This car has never seen snow. Since I've had it, it almost never sees rain, nor do I drive it after dark. I've had the same washer fluid in it for 4 years... And yes, that's original paint you see there...

 

Uh oh, easy, I think he's thinking what we all are (or, were).. that the problem is very minor and that you were nit-picking about it. But the problem's really that bad? Strange. Think your throttle cable is binding anywhere? If you disconnect the clip from the throttle body (be careful it doesn't fly across the room and disappear!!), is the butterfly hard to work, or does it stick? Is this problem temperature related, or does it happen all the time?

The fact that your car is the only one that holds it's rpm would seem to eliminate the possibility of any kind of throttle-holding device, whether electrical or mechanical, on the car. Otherwise, other people would have the same thing happen.

Could you have a leak in the intake tubing, in particular, around the MAF sensor? A leak at the IAC valve (not tightened in all the way)? Ever remove your plenum or TB to clean it, and you reused the gaskets, and maybe they're not sealing right and sucking air in from somewhere they shouldn't (along the lines of what Doward mentioned)? TPS voltage adjusted correctly? What voltage does the TPS show at idle? Did the floormat slide up behind the gas pedal and it's holding the gas down? (Happened to a few of us here, but the RPM's never dropped, just stayed constant)

 

Ok, first I gotta say - That is a DAMN FINE LOOKING CAR! Easily the best looking (and cleanest exterior) 3rd gen I've ever seen - and the fact it's a 'bird makes it all that much more special

As for your rough idle, the leaky valve seals will cause that - especially if it's a very small leak (as little as you drive it, I'd say the seals are probably brittle, and need replacing)


I'm re-re-re-reading the posts... I think I understand better what you were saying. You tap the gas, and it hangs at whatever peak rpm it hits. Can you try this for me?

Hold the clutch, with the tranny in neutral - tap gas, report what happens.

Keep tranny in neutral, and release the clutch - tap gas, and see if it does the same thing.

Why?

With the clutch pushed in, you PHYSICALLY disconnect the tranny/engine. When the tranny is in neutral, you are spinning the tranny, just without engaging any gears.

My current guess is as follows -

I pulled my throttle body today, noticing a torx headed screw is used as the throttle stop. Try turning that in/out (whichever closes the throttle blade) to force more use of the IAC from the computer. Unless I miss my mark, when you hit the gas, and release quickly, the throttle blade are NOT completely closed, and allowing air around the throttle blade, and holding your rpms - until the computer realizes the tps voltage has dropped (figuring you to not be on the gas) and closes the IAC almost completely. Then your revs come down, as the air that normally would be coming through the IAC (now closed) is coming through that "gap" in the throttle blade...

Did you follow that? That's my current best guess, anyway....

P.S. - please disregard any singular/plural errors. I'm so damn used to my QJet - 4 blades to deal with, instead of just one.

 

Doward: I will try as many combinations of clutch/no clutch hot/cold conditions as I can, and report back. I've never removed the TB, but I've cleaned it a few times while it was attached to the intake manifold. I know of the stop screw that you mentioned, and at this point, it is run all the way in, IE the blade closes completely.

I will double-check my TPS voltage, and re-learn the position into the ECM. I don't believe that there are any sticking throttle cables, but I will double check the cruise control module. I don't think I've ever tinkered with it yet.

I replaced my stock intake assembly with a custom 3" stainless steel pipe that connects between the MAF and the throttle body. The MAF is directly connected to my custom-made K&N filter element. (By custom, I mean that I took a drop-in round filter, and made stainless steel endcaps, with 3 long screws running inside the filter to hold everything in place. On one endcap is a hole with a 3" pipe on it that directly attaches to the MAF. The filter, IAT and MAF are located in the front driver's side of the engine bay, right behind the headlight.

Also, to double check what information I have here, what should my IDEAL TPS voltage readings be for closed and WOT?

PS: Thanks for the complements on the pics! Those were last years photos. This year, I have a thin pair of silver pinstripes that follow the upper part of the body line, to match the silver along the bottom of the car. The only thing missing from my car is the A/C. The previous owner deleted it, because it had t-tops. I even have the automatic mechanical dimming mirror, and the 6-speaker factory sound system. There is a slider on the upper dashboard that controls the subwoofer gain. I also added 2x6 KC driving lights directly under the front turn signals, and I am controlling them with the factory TA dash switch. I also still have the spare tire inflator cannister, although I have used it once, and it is now empty, so I carry an electric air pump... *grin* All this and only 58k miles on her...

Here is my other 'baby':

 

You know, I thought about it some more... and if the stop is all the way in, then that would be holding the throttle blade slightly open... would you double check that? try turning it out, all the way and see what happens.

 

AHH! You cursed me! My car's doing it too, now.

I installed a 180º Thermo, and did a MASS cleaning on the throttlebody, and the IAC part up top. THAT IS ALL I DID - I firmly believe your throttle blades are no closing properly. When I get a chance, I'll adjust mine - and see if it fixes the problem.

holds like 2 seconds @ 3k rpm, between shifts, right?

 

Yeah, something like that! It depends on how tight I wind it up before upshifting. I mostly drive the car like a gramma'. I will try and adjust the stop screw myself this evening. Am I looking to keep it open just a tad, or closed tight? I would think that if the IAC is sluggish, that I would want more of my air to pass through the plenum...

What do you have for optimum voltage measurments on your TPS? I have a couple of printed publications giving the voltage, but I want to double-check.

 

just about all manuals I have driven do this. They hold for 1-3 sec's so the shift will be smoother. it holds the syncros at a steady RPM longer.

Hell my old 83 ford truck even did that.

 

No no, I thought I knew what he was talking about too, until my own car started doing it. It literally drops to 3k rpm, holds 3k, then continues the drop... it's friggin' strange. Haven't had time to adjust the throttle yet....

 

See, I'm NOT crazy... *grin*

 

Just set voltage at idle, 0.55 volts, +/- 0.05 tolerance. Computer hits PE mode at a certain percentage over idle voltage; I think 80% over? Idle voltage is all you need to set. What's yours set at now, did it fall out of adjustment?

 

Ok, well my paperwork says .55vdc +/- .05 as well, that that is good.

I have set my throttle stop so that the blade is just a little open. I'd guess 2 to 3 degrees from closed shut. I then set the idle TPS voltage to +0.553vdc and the WOT voltage is +4.384vdc, which appears to be within spec.

It got better. My idle is less rough, yet it is idleing at about 900rpm instead of 600. There is also more pull in third gear with the pedal to the metal... I guess this is an added bonus.

It still holds at around 2000rpm, and now slowly drifts back down to idle. I've run out of daylight at the moment, so I'll probably dick with it tomorrow evening, and try closing the blade gradually, and re-calibrating the TPS.

 

What should the WOT voltage of the TPS be? I'm reading around 4.1vdc when my idle voltage is .55vdc...

 

Back from the dead!!! Turns out my problem from EIGHT years ago is a sticky or slow deceleration valve. (Yes, that magical non-existent device mentioned in my original post). Despite my best efforts to try and clean it, it still causes unwanted RPM hangtime. I've disabled it for now while I cannot seem to find a replacement. Anyone have a line on somebody who has one left in stock, or is it safe to leave the damned thing disabled?

 

What the hell is a "deceleration valve"?

 

LOL BackInBlack- that may be the longest problem that we've got a solution for *ever*! The decel valve was on 5 speeds, not sure exactly what it did, but it was for emissions; if I remember correctly, it stuck up over the passenger side exhaust manifold?

When I was rebuilding my car using an '84 parts car, I had a guy come up to me, he had been trying to find that valve forever for his son's car and couldn't find it. I let him have it for free b/c I didn't need it- but that was back in '96... if it was hard to find then...

How'd you find out that was the probem??

[edit] Was this part on all 2.8's w/5 speed? Or could yours be a California emissions car (and the part isn't needed)?

 

From what research I've done, it appeared ONLY in the 1986-88 Firebirds with the 2.8 and the 5-speed. The V8 with the manual did not get one. I contacted GM, who contacted the folks who buy up all the old stock that GM gets rid of, and not a single dealer in the United States has one in stock. Apparently they were discontinued 15 years ago +/-. My only source will be junkyards.

I don't think I have a CA car. I'm the second owner, and the first bought it from a local dealer around here. Not sure if they ship CA cars to the East or not...

From what I understand, the valve is supposed to lean out the A/F so that you don't dump unburnt fuel into the cat and explode it. Not really interested in that scenario by any means. I'd hate to junk a perfectly good car for lack of a $10 part...

I figured it out accidentally when I was leisurely paging through my Chilton's manual. I was looking for any decent vacuum diagrams so I could start checking and replacing my lines. I came across a cutaway diagram of the deceleration / anti-backfire valve. I understand what it does, and can certainly see that if it leaks, it will cause high idle and whatnot. In my case, I disconnected and capped the vacuum control line, and my RPM hang-time immediately went away. Now when I release the gas pedal, the tach actually comes back down like it's supposed to.

I did drive it a couple of miles with the valve disconnected, and it seems ok, (other than me now mis-timing my shifts), but I don't have any desire to damage anything. As far as my original post goes, I think I'd rather keep the 2.8 in this car. I feel that it has more value with the original 25 year old engine with under 80k miles on it to match the original paint.

It's located between the alternator and the EGR valve, screwed to the alternator bracket.

 

And just for fun, here is the aforementioned now 25 year old paint...

 

I don't think your research is quite correct... The decel valve is in the 3.1L LH0 driveability and emissions section of my '92 FSM.

As for not having one, I put my T5 in back in 06 and I have never had one, being my car was originally an auto. There are only 2 reasons I need to replace my catalytic converter... 1. The guts inside busted loose when I cut the crossover pipe off to replace the pipe, and 2. I'm going to need a second one. I've never had any problems without it, but then, my ECM automatically holds the engine speed high because there seems to be an issue with the IAC (and always has been even though the wiring seems to be good and I've replaced the IAC and ECM 3 times).

 

Well, I had called GM and they told me that the particular valve I have was only used on the f-body 2.8 for those years. The 2.8 Fiero either had a different model or none at all.

 

Yeah, I was thinking of the other book...

Either way, you should be okay without it.

 

Wow that's some awesome paint!! Yeah I think you're OK without the valve- the car drives better now that it's disconnected? As long as you're tuned-up (and not running rich already), I don't think any extra fuel will shred the catalytic convertor.

I know what you mean about junking the car for a $10 part... I can fix pretty much anything but if it's a special part (especially things like suspension that can't really be fabbed up) that's not being made, and the cars aren't in junkyards anymore... well lets just say that running out of options isn't good

 

A new answer to an old question:

After fighting this same problem, and with the help of some of the people on this forum (thanks!) I found what people were referring to as a Decel Valve:



It's marked with the arrow. I plugged the vacuum line going to it and it the idle speed drop is much faster now.

One other thing. What were referred to as "dashpots" are called vacuum pull-offs. One is used for the automatic choke (I can't remember if it makes sure the choke blade opens fully, or slows its opening, but it's linked directly to that blade.)

The other controls the speed at which the secondary air door opens. If you wanna surprise the hell outta somebody, remove the arm that goes from the pull-off to that air door. The first time they stand on the gas, they get instant 4-barrel...right now! I'd done that to a Monte Carlo (the same one I race, as a matter of fact) when I put the first iteration of the 406 in it. My wife decided to take me for a ride in my favorite car (I'd had heart surgery and couldn't drive, nor do much of anything for a while) and she thought I'd enjoy feeling the car run up thru the gears. She'd never driven it with the 406, nor did she expect the secondaries to open up immediately. It barely made it into 2nd before she jumped off the gas and looked at me like a terrified puppy. I felt much better after that. It was great therapy.

Btw, just because I'm a pedantic SOB, I'll mention that dashpots are oil-filled devices, very much like shock absorbers. SU (British) carburetors use 'em to modify the size of the venturi in front of the throttle butterfly. Basically the same thing as the secondary air door on a Q-jet. Matter of fact, they were probably invented by the same sadistic SOB. I first learned about dashpots when I started working on mechanical adding machines that had been made in the first half of the 20th century. Yeah, I'm old. They used 'em to slow the machine down so that the adding mechanism operated properly.

That's enough.