1979 Super Beetle L-jetronic runs rough on hot restart

[jamclea77044]

Hello forum,

I am working on a 79 fuel injected super beetle. Just finished installation of a rebuilt long block (rebuilt short block, new heads). Upon reassembly, I smoked the engine and corrected all vacuum leaks. The engine starts great when it is cold, warms up exactly as expected and drives perfect through warm up and when it is warm. After it is shut down and restarted within a few minutes the engine idles low/rough. It'll run rough for maybe 5 minutes gradually improving until it is running perfect again. I've gone through every component in the fuel injection and each one seems to be within spec and operating correctly. Plugs, cap, rotor, wires all new. Also I've tried a new coil. Pertronix ignitor installed.

I'be been thinking I might have a lean condition on the hot restart. The engine will smooth out perfectly if I "trick" the computer into richening the mixture. I've done this 2 ways: 1) by pushing open a little the flap of the air flow meter. The idle immediately picks up and smooths out. By pushing open the flap, the ECU should be thinking more air is going into the engine thereby increasing the fuel. 2) by removing the CHT, cooling it with compressed air and grounding the sensor (not the wire) with a jumper. The engine will run perfect when restarted. In this case the ECU will think the engine is cooler than it is thereby adding more fuel,

As I've stated, I smoked the engine so I know I do not have any significant vacuum leaks causing the lean condition. It is my understanding that the AAR is supposed to be closed after the engine is warm and doesn't open on hot restarts. Regardless, pinching the hose from the AAR makes no improvement during hot restart. So it is not the AAR causing the lean condition. Pinching the hose from the deceleration valve makes no difference. The fuel pressure is right on spec - 38 to 40 psi with pump running and engine not running, 32 psi with engine running, increases from 32psi to 38psi with engine running and throttle blipped and once engine shuts off drops to 14psi after 10 minutes. Removing the vacuum line from the FPR does help the condition to some degree as the fuel pressure increases to 39 psi which would richen the mixture a little. I've installed a block plate on the EGR pipe at the throttle to take that out of the system. I've adjusted the throttle switch to several different positions with no improvement to hot restart. I've checked the resistance from most of the wires from the ECU to the components, including the grounds, which are all good.

Regarding the TS2, I thought maybe the new one was doing something funky after a short heat soak so I purchased 2 new ones which run exactly the same. They all read about the same resistance hot and cold. And I've checked the resistance from the ECU.
I do have the 1" extension installed under the TS2 but tried adding a longer fitting on top of that (thinking was to have the TS2 a little cooler to help richen the mixture).

Here's 2 clues that I have not been able to reconcile: 1) this engine will not run when the TS2 is disconnected (either the terminal connection or the ground through the sensor). the engine will essential turn off - no stumble, nothing - almost like power to ECU or injectors is cut. 2). I have the idle set at 1050 when engine is warm and running perfectly. If I turn the engine off then immediately restart (like 2 seconds) it will be idling lower - like 900. I'm asking myself what would cause that. The only the things that changed in those 2 seconds was the vacuum was cut to the FPR and the flapper of the air fuel meter closed and reopened. And, please note This AFM is a rebuilt unit I purchased from The Bus Depot about a year ago.

I'm spending time searching and correcting very minor vacuum leaks that present themselves when I spray starter fluid but I really don't think the problem is a vacuum leak. Also, I'm going to clean the fuel injector connector on the right side of the engine compartment. Not sure if there is a ground at the resistor pack but I'll check that as well. Not hold much hope as I don't see how a dirty ground or connector will allow the engine to run poorly on hot restart but then gradually get better after a few minutes.

Please help

Jim
Looking for input

[Tobra]

New O rings on the injectors? Had a leak around one of the injector seals on mine that took forever to figure out. The rubber boot on the intake can get cracks in it that gives funky symptoms also

I bet something is giving you unmetered air, rather than a vacuum leak