Ever since I’ve got my car on the road, about 1 year, I’ve had a slight hesitation/stumble at low rpms. I’ve tested most if not all the components according to the fsm and efi bible and everything seems to check out and is in spec with the recommended values and “all” vacuum leaks have been addressed.

The car idles fine but when trying to accelerate from low rpms, there is a stumble and hesitation. If I ‘pre rev” before letting out the clutch, it’s fine. Then when shifting, as long as I’m in high rpms, it doesn’t have any hesitation, just rips. So I did some fiddling and adjustment. The real culprit is that the car is running lean as soon as I hit the throttle, at least that’s what I think. My reasoning is, I dismounted my tps, used a screw driver to turn it to wide open throttle, and it runs like a champ but granted I’m dumping extra fuel in the cylinders at all times.

Given that my sensors are reading the correct resistance during cold and hot operation led me to my vacuum advance on the distributor. I popped off the cover and when I turn the rotor, I can turn it counter clockwise about 10-15 degrees and it springs back but I have no movement clockwise. Would that indicate that my vacuum advance is not functioning? I read somewhere that mechanical advance is counter clockwise but vacuum advance is clockwise movement, is that accurate given that I only found one source of that information? This would certainly explain the hesitation I’m experiencing, or at least I think…. For reference, the hesitation is there whether the engine is cold or hot…. No difference. Another point, I adjusted the track in the afm in case there was a ‘dead’ spot and it was exactly the same afterwards.

Appreciate the input/help as always…

Sounds like the not uncommon lean air-fuel mix. For whatever reason - changes in fuel, degrading electronics, etc. - the EFI systems don't work the same today as they did then. The fix is to add resistance to the coolant temperature signal that the ECU uses. Where to find a potentiometer is, somewhat ironically, more difficult in today's Amazon'ed world. Radio Shack and Fry's stores are not around anymore.

Blue wrote up a nice description. It's pretty simple, and you can test it in the engine bay before making it permanent.

https://www.atlanticz.ca/zclub/techtips/tempsensorpot/index.html

p.s. make sure to set the AFM back exactly where it was.

Have you tested the vacuum advance? The FSM has a test for it. Chances are with the age of these cars the diaphragm is probably leaking.

26 minutes ago, dar5052 said:

The car idles fine but when trying to accelerate from low rpms, there is a stumble and hesitation

26 minutes ago, dar5052 said:

The real culprit is that the car is running lean as soon as I hit the throttle, at least that’s what I think. My reasoning is, I dismounted my tps, used a screw driver to turn it to wide open throttle, and it runs like a champ but granted I’m dumping extra fuel in the cylinders at all times.

Wide open adds 27% more than the base setting.

Thanks @Zed Head and @Yarb. If the snip below is what you were referring to with respect to the FSMs guidance on vac advance testing, there is a lot to be desired 😂… I would hate to add resistance to the cooling system until I rule the vac advance out just because then I’m masking the problem and not necessarily fixing it. But I will keep the advice in mind once I’ve exhausted all other possible scenarios. Many thanks!

Out of curiosity, I went out to my spare parts bin and found a couple partially disassembled distributors. On one of them, I was able to spin the rotor clockwise about 10 degrees and counterclockwise 10 or so degrees. In both cases, the rotor returned to the neutral position when tension was removed. At this point, I’m guessing my vacuum advance breaker plate may be seized in position and thus I’m not getting vac advance when I apply throttle? Anyway, if someone can pop off their distributor cover and confirm if they can rotate the rotor clockwise slightly, that might help me narrow down if the distributor needs to be pulled off and gone through… thanks in advance!

@Zed Head I was surprised at the 27% increase… although I guess I shouldn’t have been seeing as how fast my fuel level gauge was going down!

I think you will find that the diaphragm is cracked. Take a pistol grip vacuum tool and test it. $20 dollar investment. Another option is 123 distributors or a Chinese distributor off eBay.

You can test it on the running engine if you have a timing light.

And you can test for a cracked diaphragm by sucking on the hose.

Edited September 10Sep 10 by Zed Head

I get nothing when I suck on the hose… (that doesn’t sound right….) if the diaphragm was cracked/torn I would be able to pull air through it.

I have a timing light but I let someone use it recently and haven’t gotten it back…