Help. Engine speed dips and stalls when changing down.

Dave Bilyk

Dave Bilyk
Supporter
Changing down when coming to a stop, when I dip the clutch engine speed undershoots and sometimes it stops. Tickover and starting is fine and if I slow right down to <1000rpm before dipping clutch it drops to tickover as normal. I opened the throttle stops to give it more air but still the same. Not sure if something is changing. Idle advance in operation. Otherwise have plenty power and cruises well at low speeds in all gears. Emerald K6 with Jenvey TB's. I will revert to a previous map in case I changed something inadvertantly. Any other ideas?
Dave
 

Dave Bilyk

Dave Bilyk
Supporter
Tablet problems, so I'm on the pc now. I see my message is truncated, so missed out the fact that it's Jenvey TB's and an Emerald K6 ecu. Recently I replaced alternator so the battery voltage is up 1 to 1.5 volts.
Changing down when coming to a stop, when I dip the clutch engine speed undershoots and sometimes it stops. Tickover and cold starting is fine and if I slow right down to <1000rpm it arrives at tickover without the undershoot. When it is hot, and stalls, it's more difficult to start. I opened throttle stops to give it more air, and rebalanced the TB vacuum but that didn't help. Tried an older map in case I had changed anything, but that had no effect. If I can't fix it this weekend, I will have to use more mundane transport to get to the Goodwood Revival . Ideas?

Dave
 

Dave Bilyk

Dave Bilyk
Supporter
Well so far, I have opened up the LP and HP filters, which were fine, changed the paper one anyway. All fuel clean and no signs of contamination or debris. Visually checked a couple of injectors both fine. One thing though, the fuel pressure was showing 2.5bar which is 0.5 bar down from normal. I adjusted the regulator back to 3bar, but it didn't affect the problem. I will try some random ecu adjustments tomorrow, at least I can troubleshoot just by blipping the throttle.
Dave
 

Rick Merz

Lifetime Supporter
Idle stop adjustment could affect your low rpm when butterflies slam shut. Worth checking to make sure idle adjustment is good and throttle bodies are still balanced side to side and one to another. Cold starting could be okay since the ecu opens the IAC so that extra air is supplied during startup but when warm the valve closes.

Rick
 

Dave Bilyk

Dave Bilyk
Supporter
Thanks Rick, no IAC valve on this , idle control is just by ignition advance. It worked fine until 2 days ago. I did improve balance side to side yesterday although I am getting half vacuum on inner tb's 6 3 3 6 3 3 6 on cyls 1 to 8, so not sure how to even that up as they are linked pairs, but its always been that way.
Dave
 

Ian Anderson

Lifetime Supporter
Hi Dave
Does the ECU have a road speed sensor?
What you describe happens with a lot of the Rover V8 transplants

The road speed sensor allows ecu to work normally until about 5 mph when slowing down and then adjusts the timing to make sure it idles correctly. It also then acts as a speed limiter on the top of the Rev range.

Without it the Rover engine tends to die on deceleration

Ian
 

Dave Bilyk

Dave Bilyk
Supporter
Thanks Ian, Interesting, although I found that I dont need a road test, just blip the throttle does the trick. At the moment I am finishing of balancing tb flows at idle. Before that I increased the timing advance which had been left at 0 so have increased to 20 with marginal improvement. Just plugging away
Dave
 

Dave Bilyk

Dave Bilyk
Supporter
Update. I have today balanced the TB vacuum as well as I can 5 4 3 5 4 4 4 4. Increased advance around the tickover speed, set injector map settings so that it idles well open loop, but still the undershoot. A chart from the data log shows the effect clearly, engine rpm dipping to nearly 400rpm before overshooting to 900rpm before settling to the tickover around 750. Needs a little damping one way or another. I want to fix it as it's the only thing stopping me from taking the car to Goodwood. Any ideas guys?
stall.png
 

Neil

Supporter
FRom a control loop standpoint that overshoot implies too much gain/too little phase margin in your control system. If it was an alalog control I could help you but digital.... no way.
 

Dave Bilyk

Dave Bilyk
Supporter
With you all the way Neil , been wondering how on earth I can reduce the gain. At first I thought the throttle snaps shut very quickly, but it's so much faster than the engine response that it wouldnt be the driver for this. I am currently checking for slop in the spindle connection in the throttle body pairs.
 

Dave Bilyk

Dave Bilyk
Supporter
Update, well that was a full working day, at least I have eliminated a few things. Including yesterday, having balanced vacuum as well as I could, checked fuel system cleanliness, checked for slop in the Jenvey spindles, checked ignition tdc datum, revised tickover advance settings and fuelling, tightened manifold bolts in case of vacuum leak, I dont know what else to do, maybe a compression check but if I can't sort it tomorrow, I will need to go down to Goodwood in something more mundane
 
Have you tried raising the base idle speed to around 850 a 900rpm. I have found ITBs like a little higher idle speedand around 18 to 20* ignition timing.

Also the ignition timing values in the map BELOW idle speed need to be the same or sometimes higher values to help bump up engine torque as loads go on the idle end of the rpm..

Does the idle suffer at all when cooling fans are on for example.. If so that would point to a timing issue.
.
With a very low idle speed the cylinder pulses could be effecting low rpm vacuum balance a little as well
 

Rick Merz

Lifetime Supporter
Ricky, Agree with raising the idle speed slightly, depending on the cam it may have to be raised even more. My engine wont even run below 1050 rpm.
 

Dave Bilyk

Dave Bilyk
Supporter
Thanks guys, I will try raising the idle speed and raise the timing values below. The idle maintains really well from cold to hot with cooling fans and cold starting is good without using any throttle. I already increased the idle timing from zero where it was to 20. I think the idea was that the idle control increased advance to maintain the idle.
 

Dave Bilyk

Dave Bilyk
Supporter
Update, this morning started the engine, hunting idle speed, masses of black smoke, then neat fuel vapour, finally settling down to a normal tickover! I think this was fuel pooled in the manifold/ engine from the difficulty of getting up the garage gradient the previous day. Set idle to 900 and this does help to reduce the stall. I moved the car outside and checked for loose wires etc. With a friends help, I tried various ideas related to idle ignition and fuelling but our adjustments were not that effective, although things seem to have improved by themselves anyway. At some point on a restart, the speed started hunting again, no black smoke or vapour, but even at 1500rpm on a steady throttle, large regular swings in speed. This I believed to be fouled plugs and taking them out for a clean helped enormously, so I went out for a drive, good power but still a little stumbling during low speed downshifts.
What I did notice was that the idle advance correction did not work as expected on several occasions, so that is what I will be looking at in more detail, but later as I don't have time this week. So we will shed a couple of cylinders and take our SLK300 to Goodwood instead, it's too far to risk taking the 40 with these issues.
Dave
 
Is your system using an O2 sensor and running closed loop feedback ?

If it is going rich enough for black smoke, there could be an issue with the fuel programming /O2 sensor feedback.

I have had a bad O2 sensor in a holley system send the car so rich it was un drivable due to the fuel mapping trying to compensate for the dodgy sensor.

In your case you drive along and all is sweet. You let off throttle to coast and as your stop the idle drops / hunts

This is after the fuelling has been cut or reduced while coasting. Car will usually be lean in that condition, then the idle needs to compensate for the lean coasting and then adds fuel.

If it is running closed loop, can you easily switch to open loop for idle? It may be that the hunting is a result over and under fuelling chasing the O2 sensor or a result of the fuel cut or reduction when letting off throttle
 

Dave Bilyk

Dave Bilyk
Supporter
Hi Ricky, yes a wideband O2 sensor in one collector, with closed loop feedback, although for setting up, I change to open loop so that it's not far away from where it should be, then change back to closed loop.

I am convinced that the black smoke was fuel pooled in the inlets and fouled plugs from the previous day, the entrance to the garage is an uphill slope, so having messed about at idle all day, then stalled a couple of times getting back into the garage, the plugs were fouled and there was a lot of fuel in the inlets. So in the morning on cold start, with cranking enrichment, it made clouds of black smoke, then suddenly cleared and idled sweetly again.
I have monitored the sensor both with the Emerald Data Logger and a separate lambda gauge and O2 sensor readings are as expected, ecu maintains target from idle.

yes, I drive along and all is sweet, but to be exact, if I throttle off and dip the clutch at 1500rpm+, the engine will undershoot idle and sometimes stall. If I throttle off and dip the clutch at 1000rpm or less, it doesn't undershoot and idles ok.

Yes, I like that idea, switching to open loop would have told me if the hunting was driven by the ecu or not, I will try that if it hunts again.
The hunting and the stalling seem to be separate issues and once I cleaned the plugs, it didnt hunt again, but it still undershot idle target when I dip the clutch at 1500rpm.

On thing that seemed to work inconsistently was the idle control by ignition advance, I had set idle to 900rpm, programmed to the ecu but it still idled at 500rpm, then another time it went up to 900rpm as expected, so I need to look more carefully at that, and get a new set of plugs, or two or three sets as my friend suggested.

I don't think there is a vacuum leak, but ultimately might strip to replace gaskets. I nearly did a compression check but was running out of time, I might do that later, but engine pulls well so I doubt it's that. Looking like fouled plugs, not so good ecu settings resulting in poor idle control, but can't be sure just yet.

On a lighter note, the guy who came to help me has a '69 Corvette and does his own road tuning which is why I contacted him for this, he is installing an LS3 with forged components, looking to make 1000hp on twin turbos.
Dave
 

Dave Bilyk

Dave Bilyk
Supporter
Update and other fun!
On replacing plugs I thought that checking the old ones with a megger (high voltage insulation checker) might be informative, and it does indeed work as the cylinder producing black smoke (sooty plug) showed a zero reading, with one other plug giving a low reading and other 6 maxed out. So with a new set, much improved, and no stalling but still hunting at idle as though there is a vacuum leak.
Now on the jenveys each TB has a throttle bypass needle valve, which I had taken to be for fine tuning of idle. The jenveys are 4 pairs and to balance each couple it seems necessary to open one of the needle valves enough to balance vacuum. This had enabled matching to an accuracy of 1 inch compared to 3 or 4 inches with all needles closed. So I closed them all, raised the throttle stops to get idle speed back up and the hunting has stopped. However I am now getting a backfire on acceleration, which I doubt is a direct result of that, something else may have happened, what do you guys think? I will recalibrate throttle sensor first and see what that does, and have a look at valve gear just in case.
On Sunday before I did this, I went for a run with three Cobras and an Ultima which was great fun, but I suddenly found the gear lever go soft and stuck in third on the way back. Weld failure on the rod change selector shaft clamp, so was easily fixed once I got home. If I only had one gear, third is best, it starts ok from a standstill, will crawl at 12mph and go to 100mph, so I still enjoyed the drive home! Giving away around 200hp to the twin turbo LS Ultima, it was fun trying to keep up!
Dave
 
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