RPMs dipping below idle during shifts

[dtsol]

Having an issue with my 87 Carrera cab. When I take my foot off the gas to shift, right before blipping the throttle, the RPMs drop below idle level. It’s worst when I’ve just started driving, to the point of almost stalling a few times. Eventually when things are really warmed up, it’s gone or only very slight.

I searched but couldn’t find anything with this exact issue.

[proporsche]

try to put your A/C on if it dies more often ..i would start with basics like adjusting your fuel mixture...just one 1/2 turn with 3mm allen wrench...this way..hopefully your plug is no longer there..for that you would have to remove the air flow meter as you can see on this pic..so, counterclock.1/2 a turn......

[walt]

I had this very problem. A very minor adjustment to the fuel mixture as described above fixed it. Keep track of your starting position.

Of course you may be compensating for a vacuum loss somewhere in the system so look for this first.

[dtsol]

Awesome--thanks for the suggestion. You're suggesting I turn the mixture screw counter-clockwise--so from what the the shop manual says in your pic, that would mean my mixture is too rich. I'll check it out when I get a chance and report back.

[autojack]

Sounds similar to the issue I had recently. My CHT sensor had gone bad. Check the resistance with a multimeter when it's cold, and then when it has warmed up. Correct values can be found here: Verify my CHT sensor test results

I would do this before playing with the mixture, as it's a simple test that doesn't have the potential to introduce a new problem or variable.

[FullThrottleN.]

From 1) to
1) Check injection system 4 correct air flow, search air leaks, causing idle problems
2) check idle valve 4 correct operation?
4) if necessary replace bad rubber / seals / idle control valve
4) Set adjust Idle / Co

If the idle is still not working correct check air flow meter, ignition, ecu

80% of idle probs are caused by incorrect air flow (air leaks) and old idle control valves.

[proporsche]

dtsol..exactly your are most likely running a little rich...you can even try this
Once the car is in normal operating temperature.put your A/C on ..car sitting, step on the throttle and let go, if the engine has the tendency to die, make it leaner until you get the desired idle decent..it is a old trick i have learned in the 1980`s;-))

Ivan

[dtsol]

For those suggesting a vacuum leak—sorry for the dumb question, but I’m new at this. Is there a simple way to check this?

[Pedro '84 Coupe]

If you don't have access to a smoke machine (there are links on YouTube on how to make one) you can use some other methods such as spraying carb cleaner on your hoses or using a propane torch near the hoses to see if there is a change in engine speed. I personally don't like these two because of the small chance of sparking a fire. However, I have sprayed carb cleaner on hoses before I had access to the smoke machine and no fires

[Pedro '84 Coupe]

Some of the items I changed to address unmetered air and/or vacuum leaks included replacing: as many vacuum lines as I could; oil breather hoses; intake manifold gaskets and plastic spacers (you can reuse the old ones if needed, just make sure they are flat on both sides); the rubber piece that connects both halves of the intake manifold; the rubber accordion boot deal at the air flow meter; and throttle body gasket. Another area to check is the fuel injector o-rings. I'd also try cleaning your idle control valve even though that wouldn't necessarily be a vacuum issue, unless the hoses that connect to it are bad.

[dtsol]

I tried the air conditioner test. The RPMs increased a little at idle--less than 200 rpm. But the problem seems to have resolved itself. I wiggled everything I could touch in the engine bay and pushed down on the plugs to make sure everything was tight. All seemed good. Hoses looked good. Car's running great now. No longer dropping below idle more than maybe 100rpm when cold. The weather has warmed up here considerably in the last week. I wonder if that would have anything to do with what was going on?

[autojack]

I still think you should test the CHT sensor

[dtsol]

I just ordered a multimeter—I’ll check it out.

[nene]

I have been experiencing this issue on my 86 and to date, I have not resolved it. New CHT, adjusted Wiper Arm including the AFM.

[walt]

Have you tried suggestion in post number 2?