When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Dear friends, I have a strange issue which I cannot retrieve on our 928 forum..
I have a Euro CIS Kjet of 1980, 3sp AT, 4.7L 300HP Cold start OK. Coldstart fuel pressure = 1.2bar (17 Psi) Warm start OK. Warmstart fuel pressure = 3.4bar (49 Psi) Acceleration is smooth and with sufficient power. System fuel pressure = 5.8bar (84 Psi)
However , when engine is fully warmed up, the idle is not really consistent. Engine stumbles a bit. But my rpm @ idle remains @ 800-850.
Any idea where to look ?
Remarks : the following components have been renewed : injectors, WUR, Fuel pump, Fuel accumulator
Put an electronic tach on ignition wire number 1 to see what your RPMs are actually doing when you hear the car having an incostent idle. My timing light had one on it. Should be more accurate than the gauge. You might find that the idle is actually hunting more than it looks like it is on the gauge which may point to a vacuum leak that you'd probably only notice at idle. Smoke testing the intake would be another good test to confirm or rule out a vacuum leak.
You could also turn the idle screw just a little and see if raising the idle 500 rpm makes it more stable.
What's your ignition timing at idle and at 3,000 rpm (with vacuum lines off the distributor and then with them on the distributor)?
Put an electronic tach on ignition wire number 1 to see what your RPMs are actually doing when you hear the car having an incostent idle. My timing light had one on it. Should be more accurate than the gauge. You might find that the idle is actually hunting more than it looks like it is on the gauge which may point to a vacuum leak that you'd probably only notice at idle. Smoke testing the intake would be another good test to confirm or rule out a vacuum leak.
You could also turn the idle screw just a little and see if raising the idle 500 rpm makes it more stable.
What's your ignition timing at idle and at 3,000 rpm (with vacuum lines off the distributor and then with them on the distributor)?
i will certainly recheck my timing, it should be 26 degree btdc @ 3000 rpm
Could too lean also cause this ?
Remark : when Cold starting my engine @ 18 degree Celsius , rpm is 900. Shouldn’t this be at least 1000 rpm?
i will certainly recheck my timing, it should be 26 degree btdc @ 3000 rpm
Could too lean also cause this ?
Remark : when Cold starting my engine @ 18 degree Celsius , rpm is 900. Shouldn’t this be at least 1000 rpm?
Yes, cold start should have a higher idle and that's managed by the cold start injector at the front of the spider intake and by the Auxiliary Air Valve that opens when cold and closes down as it warms up.
The plug on the AAV should have 12v whenever the key is in the run position. The plug on the cold start injector should have 12v whenever the starter is cranking. This is the only plug with 3 wires as it's hooked in series with the coolant temp sensor below it. When that sensor detects the engine is cold, it allows the current to pass to the plug on the cold start injector. When it senses it's warm, it breaks the ground connection so the injector doesn't fire.
The third plug that looks like the other 2 goes to the WUR (Warm up Regulator). That one should also have 2 wires and 12v whenever the key is in the Run position.
Yes, cold start should have a higher idle and that's managed by the cold start injector at the front of the spider intake and by the Auxiliary Air Valve that opens when cold and closes down as it warms up.
The plug on the AAV should have 12v whenever the key is in the run position. The plug on the cold start injector should have 12v whenever the starter is cranking. This is the only plug with 3 wires as it's hooked in series with the coolant temp sensor below it. When that sensor detects the engine is cold, it allows the current to pass to the plug on the cold start injector. When it senses it's warm, it breaks the ground connection so the injector doesn't fire.
The third plug that looks like the other 2 goes to the WUR (Warm up Regulator). That one should also have 2 wires and 12v whenever the key is in the Run position.
So perhaps I should check whether :
- AAV opens enough at coldstart
- coldstart injector sprays enough
- there are no vacuüm leaks
- timing is correct
So perhaps I should check whether :
- AAV opens enough at coldstart
- coldstart injector sprays enough
- there are no vacuüm leaks
- timing is correct
it is only at warm idle that I have this issue
Seems like a good list. Sometimes things show up when the engine is warn and they expand to open up leaks that aren't present when they're cold and things are contracted. Since it happens when hot, maybe smoke test the intake when it's hot and see if any smoke emanates from anywhere.
Seems like a good list. Sometimes things show up when the engine is warn and they expand to open up leaks that aren't present when they're cold and things are contracted. Since it happens when hot, maybe smoke test the intake when it's hot and see if any smoke emanates from anywhere.
Something interesting I discovered : I cold started engine @ 18 °C where directly after start rpms climbed steady and slowly from 500 rpm to 900 rpm, so not directly high rpm. I then directly cut the engine so nothing has warmed up. Then after 3 hours, I cold started again and now rpm was directly 900 without hesitation.
Could a bad working fuel pump relay also cause several idle issues ? Or is the relay just working or not at all?
I know that fuel pump relay also sends signal to AAV...so if there is something wrong with my fuel pump relay, the AAV will not properly work...
Strangely enough all my fuel pressures warm & cold are correct. Car is accelerating fine, warm start is also good....so what am I missing here ?
Something interesting I discovered : I cold started engine @ 18 °C where directly after start rpms climbed steady and slowly from 500 rpm to 900 rpm, so not directly high rpm. I then directly cut the engine so nothing has warmed up. Then after 3 hours, I cold started again and now rpm was directly 900 without hesitation.
Could a bad working fuel pump relay also cause several idle issues ? Or is the relay just working or not at all?
I know that fuel pump relay also sends signal to AAV...so if there is something wrong with my fuel pump relay, the AAV will not properly work...
Strangely enough all my fuel pressures warm & cold are correct. Car is accelerating fine, warm start is also good....so what am I missing here ?
This sounds like AAV issues and the reason I say that is because my car had an unusually high cold start idle of around 1,800 rpm. I attributed this to the additional air being forced in by the supercharger and started playing around with an inline restrictor between the AAV and intake. If I made the restrictor too small, the car would start then die. If I kept it running, the idle would be low, but eventually stabilize and once fully warmed up, be completely normal. I started boring out the inline restrictor 1/64" at a time and now have it perfect. Cold start idles at 1,200 and is very stable then lowers as the engine warms up to the 900 rpm I have my idle set at. Does this every time now. It sounds like your AAV may not be opening all the way when the car is cold. Once you've started it, you've probably generated enough heat to where the next time you started it, it wasn't exactly the same as true cold start even though you had shut it of pretty quickly.
There's an easy way to test your relay theory though. Jumper the relay so the fuel pump runs all the time and see if it behaves any differently. That will tell you if it's a fuel supply side of the relay that has an issue - I doubt it since you say the car runs fine once warm. When the relay doesn't work, the car really won't run at all.
Interesting theory about the relay and how it relates to the AAV though. You could cold start it with the relay installed and see if you're getting 12v to the AAV plug and if it's consistent or drops in and out. The AAV should be fully open on cold start, so voltage or no voltage there at cold start shouldn't effect that. The voltage there heats it up and closes it. Sibce you say the car runs fine warm, I believe the AAV is closing properly, but that it may not be opening properly when cold.
If the cold start injector wasn't working, I don't think the car would start well at all when cold and it would take multiple restarts to get it to run.
What's your ignition timing at idle and at 3,000 rpm (with vacuum lines off the distributor and then with them on the distributor)?[/QUOTE]
Ive always wondered why its important to remove the vacuum line when setting the timing at 3,000rpm. At 3,000rpm there is no vacuum - so why disconnect ?
This sounds like AAV issues and the reason I say that is because my car had an unusually high cold start idle of around 1,800 rpm. I attributed this to the additional air being forced in by the supercharger and started playing around with an inline restrictor between the AAV and intake. If I made the restrictor too small, the car would start then die. If I kept it running, the idle would be low, but eventually stabilize and once fully warmed up, be completely normal. I started boring out the inline restrictor 1/64" at a time and now have it perfect. Cold start idles at 1,200 and is very stable then lowers as the engine warms up to the 900 rpm I have my idle set at. Does this every time now. It sounds like your AAV may not be opening all the way when the car is cold. Once you've started it, you've probably generated enough heat to where the next time you started it, it wasn't exactly the same as true cold start even though you had shut it of pretty quickly.
There's an easy way to test your relay theory though. Jumper the relay so the fuel pump runs all the time and see if it behaves any differently. That will tell you if it's a fuel supply side of the relay that has an issue - I doubt it since you say the car runs fine once warm. When the relay doesn't work, the car really won't run at all.
Interesting theory about the relay and how it relates to the AAV though. You could cold start it with the relay installed and see if you're getting 12v to the AAV plug and if it's consistent or drops in and out. The AAV should be fully open on cold start, so voltage or no voltage there at cold start shouldn't effect that. The voltage there heats it up and closes it. Sibce you say the car runs fine warm, I believe the AAV is closing properly, but that it may not be opening properly when cold.
If the cold start injector wasn't working, I don't think the car would start well at all when cold and it would take multiple restarts to get it to run.
First results :
AAV gets 12V , here is a picture of AAV
in open status before the coldstart
Is the open status OK ? Outsider temperature is 23 degree Celsius.
The coldstart injector seems to work
also.
So I still need to check my timing.
for the opening aperture difference between the 2 AAVs used on K-Jet vs L-Jet cars and make sure yours has the right one for your fueling system, and that it's opening the correct amount. The K-jet one opens further than the L-Jet one allowing more air to pass through and a higher idle.
06-11-2020 | 03:25 AM
erratic idle but rpm stable ?
