Remove Deceleration valves

[Bitsyncmaster]

Since I had my old throttle body with the bad deceleration springs, I decided to test what it would do with the valves sealed shut. I just used a rubber washer under a #10/32 screw and nut to close the openings in each throttle plate.

I've only done testing in the garage but it looks like the only possible change is the RPMs may undershoot just a little (with my micro ECU) when you take your foot off the gas. I think that makes sense since with the valves working it would open them at that time and result in "more throttle". There were a few people that tried this 6 or 7 years ago but I don't remember what problems they had running the stock ECU.

I'm going to road test it and if that works out OK, I can modify my ECU software to fix the undershoot.

[dmc6960]

The primary purpose of the decel valves is emissions related. Though it can have an affect on keeping the engine running with a stock setup if the RPMs drop real fast. When I did this I set the idle bypass and balance screws opened up to just under the threshold of needing the idle motor at all. That theory should be no different with your custom idle ECU. Only difference is that yours can probably react to a fast dropping RPM quicker than OEM.

Hot Engine Idle Air Requirements (no accessories on) - 90% bypass screws, 10% idle motor.

This also has the advantage of leaving a large range of movement available from the idle motor for more loaded situations (engine load that is). AC and alternator (fans) load, torque converter (ATs), and even engaging first gear on a manual without using the gas pedal (assuming your fairly soft on the clutch).

[DMCMW Dave]

Since I had my old throttle body with the bad deceleration springs, I decided to test what it would do with the valves sealed shut. I just used a rubber washer under a #10/32 screw and nut to close the openings in each throttle plate..

I would weld or solder the holes. The last thing you want getting sucked through the engine is a #10/32 screw and nut.

Note that the original spring assembly is soldered in place.

[Bitsyncmaster]

I would weld or solder the holes. The last thing you want getting sucked through the engine is a #10/32 screw and nut.

Note that the original spring assembly is soldered in place.

I used nylon locking nuts. Yes the OEM has a threaded adjustment but they soldered to lock it there.

[DMCMW Dave]

I used nylon locking nuts. Yes the OEM has a threaded adjustment but they soldered to lock it there.

I've seen screws pounded into piston tops. After tearing up an intake valve. Not pretty.

[Bitsyncmaster]

I've seen screws pounded into piston tops. After tearing up an intake valve. Not pretty.

Interesting. I would not think the vacuum could pull a steel screw up from the mixture unit into the intake.

[mluder]

Slightly aside on this topic...

If I understand correctly, when the car decelerates it is normal to see a slight RPM surge as you come to a stop... This is the result of the decelerator springs doing their job to keep the engine from dying as you settle into idle?

I've never driven any other DeLorean but my own so I was curious if my car acted normally in this instance.

Cheers
Steven

[DMCMW Dave]

Slightly aside on this topic...

If I understand correctly, when the car decelerates it is normal to see a slight RPM surge as you come to a stop... This is the result of the decelerator springs doing their job to keep the engine from dying as you settle into idle?

I've never driven any other DeLorean but my own so I was curious if my car acted normally in this instance.

Cheers
Steven

That's more likely the slow idle control circuit overshooting.

[DMCMW Dave]

Interesting. I would not think the vacuum could pull a steel screw up from the mixture unit into the intake.

Lots of air flow there. It only has to get around the W pipe and then it's down hill.

[mluder]

That's more likely the slow idle control circuit overshooting.

So is the fix to replace the idle control computer?

Cheers
Steven

[Tomcio]

So is the fix to replace the idle control computer?

It could be the ECU or the motor that's slow to react.
The ECUs are known problem areas. If you are handy with a soldering iron you can fix one yourself. I've fixed quite a few of them with great success. Total parts cost was about $5.
What you need to do is replace all ICs you can see in there, six in total.
three LM2902
one LM2901
one CD4066
one CD4027

It might be a good idea to replace electrolytic capacitors as they go bad with age.
You'll need:
one 100uF 40V
two 10uF 35V
one 47uF 10V
one 1uF 63V

If the motor is slow to react... I've heard someone spray some lubricant into the air channel to free up the rotor... I guess it's worth a try.
Either way, when running correctly your idle should not overshoot. After revving up and releasing the gas the idle should drop and just stop at idle without going under and then correcting.

[NightFlyer]

It could be the ECU or the motor that's slow to react.
The ECUs are known problem areas. If you are handy with a soldering iron you can fix one yourself. I've fixed quite a few of them with great success. Total parts cost was about $5.
What you need to do is replace all ICs you can see in there, six in total.
three LM2902
one LM2901
one CD4066
one CD4027

It might be a good idea to replace electrolytic capacitors as they go bad with age.
You'll need:
one 100uF 40V
two 10uF 35V
one 47uF 10V
one 1uF 63V

If the motor is slow to react... I've heard someone spray some lubricant into the air channel to free up the rotor... I guess it's worth a try.
Either way, when running correctly your idle should not overshoot. After revving up and releasing the gas the idle should drop and just stop at idle without going under and then correcting.

This is great info - I just bookmarked it!

Thanks Tom :thumbup:

[Bitsyncmaster]

I've got my idle ECU software fixed so it seems my problems with the plugged deceleration valves will work. The road test on my 500 mile drive to DCS and back should ring out those changes. I think this software will work maybe better on cars even with the deceleration valves working but I need to test that also.

I had some hard starts which was a simple fix to run the ISM more closed during hot restarts.

The RPMs would drop to about 500 (curb idle) sometimes when stepping on the clutch. I fixed that by adding a little to the ISM setting during that condition.

I've been running the maximum delay (dip switch setting) with these tests so that should be worst case but I think I will leave it set that way since I seen no problems set that way.

[DMCVegas]

For what it's worth, I extended the striker screw and bent the bracket on the Microswitch UP so that the switch is actually triggered BEFORE Closed Throttle Position. As a result my RPMs fall slower at a much more gentle rate instead of that sudden plummet and subsequent choke. This also had the unintended consequence of not only preventing a partial stall of the engine so that power was always on tap, but it stops shocking the electrical system and the alternator. Even with the 150amp Alternator, I'd still get dim lights and an electrical drop across the car because the RPMs dropped way too low. After that modification the electrics are so much more smoother.

The valves are absolutely there for no other purpose than emissions. The idea was that the the faster the RPMs fall to Idle speed, the less emissions the engine creates. But the other problem this created is that with the delayed activation of the Idle Speed Motor/lack of constant air flow through the Idle Screws, the bypass tube is unable to supply enough air to the engine. This results in a temporary rich mixture to the engine that bumps up the Hydrocarbons. So the valves are there to pop-open via engine vacuum to feed air until the ISM cracks open to take over feeding the engine. Now if you bend up the bracket like I've done you can trigger that Idle Speed Circuit earlier and get a much gentler drop in RPMs and negate the use of those valves all together.

In fact, removing the Deceleration Valves is a tuning modification that John Lane has recommended for years on the PRV engine. They themselves take up a good portion of space and actually block airflow. Lane stated that he soldered in the holes and then would grind them down smooth to reduce any turbulence and create as much flow as possible through the throttle.

While we're on the subject, disconnect and re calibrate your armature that connects the Throttle Spool to the Butterflies. You'll probably find a good amount of slack was left there by Française de Mécanique back in Douvrin. Spin the Sockets and get them to there limits to connect with the Ball Joints so that there is no slack at all. Even if you have to preload a slight bit of tension without actually cracking the valves. Once done you're still not going to exactly start hitting the 12's on the dragstrip, but you'll find your engine is a bit more responsive.

[mluder]

For what it's worth, I extended the striker screw and bent the bracket on the Microswitch UP so that the switch is actually triggered BEFORE Closed Throttle Position. As a result my RPMs fall slower at a much more gentle rate instead of that sudden plummet and subsequent choke.

This is an interesting idea... I too have that sudden deceleration as I come to a stop at a traffic light. It doesn't do it all the time but most of the time. The engine takes a little dive into the 500 RPM teritory and them pops up above idle (1000-1200 RPMs) and then settles into idle. Do you think your adjustment might fix that?

[Bitsyncmaster]

This is an interesting idea... I too have that sudden deceleration as I come to a stop at a traffic light. It doesn't do it all the time but most of the time. The engine takes a little dive into the 500 RPM teritory and them pops up above idle (1000-1200 RPMs) and then settles into idle. Do you think your adjustment might fix that?

Do you have my idle ECU? If so I will bring my programmer and burn the new software into yours if your going to DCS. If not at DCS I can update yours my mail.

Anyone can also buy the programmer which last I checked is about $35. Then I can email the new file to burn.

[mluder]

Do you have my idle ECU? If so I will bring my programmer and burn the new software into yours if your going to DCS. If not at DCS I can update yours my mail.

Anyone can also buy the programmer which last I checked is about $35. Then I can email the new file to burn.

Hi, Dave.

I appreciate the offer but I'm still running the original ECU... I do have a spare so I might try swapping it out to see if that solves anything. If not perhaps I'll look into buying one of yours.

[Bitsyncmaster]

Hi, Dave.

I appreciate the offer but I'm still running the original ECU... I do have a spare so I might try swapping it out to see if that solves anything. If not perhaps I'll look into buying one of yours.

Your welcome to try one of my idle ECUs at DCS if your going.

[mluder]

Your welcome to try one of my idle ECUs at DCS if your going.

I wish I was going, sadly I'm not... Maybe in 2 years.

[DMCVegas]

This is an interesting idea... I too have that sudden deceleration as I come to a stop at a traffic light. It doesn't do it all the time but most of the time. The engine takes a little dive into the 500 RPM teritory and them pops up above idle (1000-1200 RPMs) and then settles into idle. Do you think your adjustment might fix that?

Most likely, yes. I've done it to a couple of other cars and it worked the same. It was a rage repair I did. I was showing the car off one day, I revved the throttle spool, and the engine dropped down to 500 RPMs or so then died. So I busted out a little wrench and a screwdriver, and that didn't solve the problem. So out of a fit of rage I just grabbed the bracket and bent it up to try and get the damn switch to be fully depressed when the throttle was closed. Started the car up afterwards and immediately saw the difference. I know it's not exactly supposed to rest on the micro switch, just press enough to trip it. But in the words of Dean Martin, "Ain't nobody got time for that." Two things here to keep in mind:


The RPMs will float down to 750± (or 1,200± when cold) from higher speeds. If you drop it down from 6K, it'll take a little less than 2 seconds. Doesn't sound like much, but it's a big difference.
The RPMs will also jump when you press on the gas pedal. Granted too I have tweaked my throttle linkage as well, but the engine will go from 750± and then jump to about 1,150± or so. Helps ensure that you don't lug the engine though.


It's free and totally reversible.