RPMs dip after a cold startup

[m_w]

I have my car running fairly well now - it starts quickly and easily both hot and cold and has a stable idle (albeit a high idle). It also drives nicely, however, I could use some help on where to look for one specific problem.

On a cold start after about 30 seconds the idle will dip and stumble. It doesn't stall, but gets close. another 20-30 seconds and the idle smooths out and the symptoms will not return (until the next cold start).

Any suggestions on where to start looking? Do you need more info? I just redid lots of the wiring around the ballast resistor on the firewall. That solved my hard-to-start-when-cold problem. Thought I'd mention that, in case its related.

[Bitsyncmaster]

It may be that is when the O2 sensor has warmed up and then the Lambda goes closed loop. I guess you don't have your idle system holding 775 RPM. Fixing the idle system would probably correct your RPM dip problem.

[m_w]

Good points, I can look into that.

I left out the (rather important) info that this is a new engine build, and the kjet hasn't been fully tuned yet. I set it to a point where the car would reliably run, while I sorted out a host of other bugs. I have the bulk of them fixed, but it sounds like I'm to the point now where I need to get the AFR and dwell correct.

[David T]

Good points, I can look into that.

I left out the (rather important) info that this is a new engine build, and the kjet hasn't been fully tuned yet. I set it to a point where the car would reliably run, while I sorted out a host of other bugs. I have the bulk of them fixed, but it sounds like I'm to the point now where I need to get the AFR and dwell correct.


Sounds like either or both:
Bad Control Pressure Regulator
Vacuum leaks
Get the idle right first as suggested. Make sure the idle switch is being tripped when the throttle arm is at idle. Close the 3 brass screws lightly. Make sure there is a plug over the mixture screw. The O2 sensor doesn't function till the motor is warmed up. Till then it is in "Open Loop". Since it is a "new" motor you should run it a while before making final adjustments. I would also check the valve clearances at that time.

[m_w]

Sounds like either or both:
Bad Control Pressure Regulator
Vacuum leaks
Get the idle right first as suggested. Make sure the idle switch is being tripped when the throttle arm is at idle. Close the 3 brass screws lightly. Make sure there is a plug over the mixture screw. The O2 sensor doesn't function till the motor is warmed up. Till then it is in "Open Loop". Since it is a "new" motor you should run it a while before making final adjustments. I would also check the valve clearances at that time.

Good points -

Control Pressure Regulator - I'm assuming you mean the Warm Up Regulator? It's new from DMCMW - So hopefully it's okay.
I can check for vacuum leaks, I have a gauge I should be able to tie in to the system and see what the inches of vacuum are.
The plug is in place over the mixture screw.
I don't have too many miles on it. Maybe it's time I take it for a bit longer of a drive. and see how it runs on a bigger trip.
My valve train is not adjustable (3.0L setup).

[Rich]

The way to check for vacuum leaks is with a smoke test. A good shop can run that for you if you don't have a smoke machine.

Vacuum gauge won't give you a reading that means anything (unless leak is massive - yours isn't) and in any case if you have a leak it won't tell you where the leak is like a smoke test will.

Did you check David T's point about the throttle linkage? It is engaging the idle microswitch every time? And manually closing the throttle butterfly at right side of manifold does not lower the idle?



I can check for vacuum leaks, I have a gauge I should be able to tie in to the system and see what the inches of vacuum are.

[m_w]

I know this is fairly old, but the solution to this problem was adjusting the fuel mixture. I had it tuned rich to start (intentional, as I wasn't sure what it needed switching from a 2.8 to 3.0 liter). and now that it is leaned out closer to spec, the idle is smoothed out at startup.

[David T]

I know this is fairly old, but the solution to this problem was adjusting the fuel mixture. I had it tuned rich to start (intentional, as I wasn't sure what it needed switching from a 2.8 to 3.0 liter). and now that it is leaned out closer to spec, the idle is smoothed out at startup.

Break the motor in and then readjust the mixture and the valves.