Jump to content

'Choke' on a 3.9 hotwire?


ThreeSheds

Recommended Posts

Thinking out loud:

As my air bypass valve has started sticking YET again ( :angry: ) - has anyone ever built a replacement which mightuse a choke cable to control the plunger (which might have a teflon cone on the end for better sealing) thereby effectively giving a manual choke and hand-throttle all in one..

If anyone could put together such a kit for less than (say) £30 (or point me to one) - I'd have one!

Roger

Link to comment
Share on other sites

They work OK without any choke anyway, but the easy option is to connect a Bosch PWM valve from something else to the MegaSquirt (if you're sensible) or a small 555 timer adjustable PWM circuit controlled from a pot on the dash if you like making things hard ;)

Why connect a cable to the idle valve when you could block it off and just add a bit of tension to the throttle arm directly, it's exactly the same.

Link to comment
Share on other sites

Forget it and buy a decent stepper motor,

Tried that - they are £70 a pop and I have gone through three in 6 years :o - I was hoping for a cheaper option now.

closing off the air will just slow the engine down.

I didn't think that they were open during normal driving - I thought that their only purpose was tickover maintenance?

The ecu controls cold fuelling via info from the coolant temp sensor.

Yes - I know my suggestion wouldn't really be a choke as such - which was why I put it in parenthesis :) it's just an idle speed adjuster with the capability of setting it quite high - maybe as much as 3000rpm with luck for use as a kind of 'hand throttle' (in parenthesis) ;)

-------------------------

Fridge - I am just ignoring your first para, partly because I don't understand it and partly because you used the 'squirt' word ;):D

Why connect a cable to the idle valve when you could block it off and just add a bit of tension to the throttle arm directly, it's exactly the same.

I thought that if I was going to the trouble of blocking it off I might as well make it into a kind of hand throttle and that it would be more accurately adjustable than anything pulling on the throttle... Might be useful when standing with lights on or jump-charging someone

Just thinking out loud... Simplest option is probably to block off the bypass hose and adjust the base idle up a bit...

:lol:

Thanks for the replies guys :)

Link to comment
Share on other sites

That really is unlucky,the few that I have replaced over the years have been fine,and they have only been replaced when they get so sluggish that they sometimes cant quite just catch the engine from stalling when pulling up at junctions etc.

They do more than idle work,they stay open during driving and only shut down to to a target idle when the vehicle is at standstill,this is to prevent stalling when gearchanging,making quick steering inputs etc.

Has your existing unit died completely ?

Link to comment
Share on other sites

Has your existing unit died completely ?

No - in each case mine have become sluggish or not able to close properly so that when pulling up the idle will stay high. Worst case is around 3000rpm but they get worse over time and this one is currently only going to about 2000 - and then not all the time. I am well versed in cleaning them and that usually helps for a while, but my experience has been that once they start to go you might as well start saving for a new one - cleaning only helps for a short time :(

A few weeks ago (when it started playing up) I tried driving around with a clamp on the bypass tube so that it was doing nothing (I had increased the base idle) and to be honest, I couldn't tell any difference in performance - however what you said about it being open during running does help make more sense of my symptoms.. :)

Another thought that I have had is to double up on the closing spring with one from an old unit I have in the garage - might try that next...

Link to comment
Share on other sites

Update:

Well I tried the fast idle control - made one using an old choke cable, a biro outer, a paper-clip (large) and a clamp-on magnet from a bicycle speedo, the whole assembly operating to open the throttle when pulled - then I fitted a blocked off bypass tube - and it works. :ph34r:

Two problems - One is that the idle control is way too coarse - it is tricky to get it right and sometimes cold starts are accompanied by it running straight up to 4000 rpm! Not good.

The other problem is that - as Ally V8 said, these air-bypass valves open up at speed, effectively giving a little more throttle area, and the car was marginally slower on the "30mph in third and floor it" test, hitting 48 by the 2nd marker as opposed to 52 with a working valve.

SO...

I set to cleaning the valve (once again) and this time I thought that I needed to clean it while it is fully out - a condition you don't normally see them in. So I connected it to the wires with another valve fitted to block the hole (and my blocked bypass pipe still in place) and would up the base idle speed to about 1500, then with a bit of 'encouragement' the plunger came right out (in little steps) as the ECU attempted to reduce the perceived idle speed. I then disconnected it and cleaned it while in the extended condition, finally giving it a squirt of aerosol copper-grease. I have never dared do this before in case it shorts out the stepper motor (can it do that? How do they work?) but I thought 'what have I got to loose?'

Next I connected it to the loom again and wound down the base_idle speed. As expected this triggered the ECU to wind the plunger back in fully, then I replaced the blocked pipe with a proper one and put the valve back in. On starting and running now all is back to normal. :)

I don't know how long it will last but all is good for now...

Interesting aside - while checking it out I did some experimenting and found the following that I never noticed before:

1. At speeds below about 3mph, on a closed throttle and in neutral, the idle speed is 750rpm.

2. Between 3 and 40mph, on a closed throttle and in neutral, the idle speed is 1100rpm.

3. Above 40mph, on a closed throttle and in neutral, the idle speed is 1500rpm.

4. When slowing (rolling without using brakes) through the 'trigger speeds' the idle speed appears to drop quite deliberately so I think that this is ECU control and not just gearbox drag.

I didn't think to check for repeatability of these results so they may be flukes and/or further symptoms of my sticky-valve problem.

I will report back later as to whether this 'extended' cleaning is more effective (lasts longer) than previous attempts.

Link to comment
Share on other sites

Interesting stuff learning how EFI actually works in practice eh ? The other point to note is that the amount of air the stepper motor has control of is quite small.Because of this it can only work properly if the engine is making power properly,ie the thing is running closed loop of the oxygen sensors,no air leaks,misfires,a decent air flow meter etc.Even a small air leak can cause big problems by the time the oxy sensors give feedback from it.It takes me around an hour to go right round a V8 checking everthing before doing final tuning etc if its a tickly or intermittent issue, often is darned obvious,sparks jumping around.Is your engine all up together ?

Link to comment
Share on other sites

Interesting stuff learning how EFI actually works in practice eh ?

Hmm... I would rather see the functional design document for it and from that understand the reasoning behind some of the decisions it takes ;)

The other point to note is that the amount of air the stepper motor has control of is quite small.

Relative to the area of the main choke - yes, but when that hole (looks like 1/4") is wide open with a stuck valve and you are sitting at the lights in central Leeds with the engine 'ticking over' at over 3000rpm, it's quite large enough thank you! ;)

Because of this it can only work properly if the engine is making power properly,ie the thing is running closed loop of the oxygen sensors,no air leaks,misfires,a decent air flow meter etc.Even a small air leak can cause big problems by the time the oxy sensors give feedback from it.

Ah well - no sensors on mine, it's a 1990 model you see :) I agree about the air leaks though, and the first thing I do if I suspect problems is block off the air bypass tube and connect up the MegaJolt diagnostics which gives me an absolute MAP reading. I have found in the past that a split vacuum tube increases MAP at tickover by about 10% and a small pipe hanging off, by about 20% - amazingly each time, with a working air bypass valve, the ECU was able to compensate! When it's all working as it should, that system is quite capable of masking significant problems, which is why the first thing I do when looking for suspected problems is block it off. Previously I would clamp the pipe, but now I have a couple of bungs (one for each end) made out of an old bypass pipe and a couple of tow-ball bolts. :)

Is your engine all up together ?

Nah! Nowhere near! The cam and timing chain are badly worn (on the old distributor, the timing varied by about 15o on the strobe) are the most significant problems, but it also had several sticky tappets which sometimes sounded awful, so goodness know what they were doing to the power! Before I laid it up 5 years ago (after an MoT failure on rust) I was planning to replace the engine because I thought it was shot - it would not start on LPG at all, and on petrol it was hit and miss. Running was rough and power was down and tickover was always a stumbling affair. Some days it wouldn't run on gas at all. :(

Then - while doing my four year rust-hunt - I did some stuff that transformed it:

  • I MegaJolted it and fitted decent ignition leads.
  • I stripped and cleaned the vapouriser (about a cup-full of black 'high-ends' came out of the bottom of it!).
  • I did a couple of cycles of oil-changes using Forte engine-flush (amazing stuff!) which almost completely cured the sticking tappets.
  • I cleaned the AFM and adjusted the base-idle speed correctly.

Anyway - now, when the air bypass system is working properly, it runs really well for an old-un :)

The tappets are mostly quiet, it starts and runs nicely on either petrol or LPG and makes good power. I am very pleased with it and the replacement engine has been firmly pushed to the back-boiler. It still is not in "good condition" though and the fact that it feels ok is more testament to the ability of these V8s to keep going through adversity than anything else.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We use cookies to ensure you get the best experience. By using our website you agree to our Cookie Policy