1.6 gti jerky when driving help!!

[Ren89 0]

I have a 1.6 gti 1990 which i have been having a few problems with. The engine has been stripped down and rebuilt and its basically at a point where everything is running ok but it is very jerky and inconsistent when driving however it idles and revs perfectly fine when stationary, i have had the AFM refurbished by ATP, replace coolant temp sensor, ignition coil and rotor arm. My mechanic has adjusted and tested the emissions to get it running properly but isn't sure that the AFM is working right even though it has been refurbished. It does struggle to fire the first few tries but will always fire up and run fine so i'm not quite sure where to go from here besides driving it regularly as it has not been used properly for a long time.

 

Any help would be appreciated 

 

Thanks Craig.

[jackherer 543]

You need to check that e.g. the coolant temp sensor resistance is correct then also check the same resistance at the ECU plug to confirm the wiring is good.

 

It's no use just throwing parts at it, you may have had a good AFM before and after refurbishing but if the wiring is no good it still wont work properly.

 

There are a few other common mistakes that affect fueling so are worth checking for, such as the bolt on the side of the inlet manifold near the oil filler which needs to be air tight or if you fitted the wrong inlet manifold gasket the injectors could be covered by it.

 

As you say it struggles to fire the first few goes it might not be holding fuel pressure. See if your mechanic has a fuel pressure gauge to test it while you try to start the engine. A one way valve in the pump should hold pressure in the rail after you turn the engine off, if it bleeds away immediately there is an issue.

 

Don't be surprised if there is more than one fault contributing to the problems you're having.

[DamirGTI 325]

Me thinks it just needs fuel mixture and ignition timing adjusting .

 

If it's rebuilt AFM , most probably it's not set up properly ... same goes for the ignition timing (best to do "by ear" on the road , rather than stationary with the strobe light)

 

Did some analysis/experimenting with the mixture on my 205 which i use daily , it has WB sensor and gauge installed permanently so that i can monitor the mixture while driving , and do the adjustments via AFM spring tension and fuel press. regulator (it's an adjustable regulator) ..

 

Anyhow , as soon as you lean out the mixture on the AFM - it starts to be jerky/shaky while driving while on-off the throttle , especially on low speeds stop and go driving with low throttle opening , creating really annoying irritating driving experience .

On the other hand , as you start to enriching up the mixture - it starts to smooth up , to a point where it's almost jerky/shaky free while driving .. some of the "jerkiness" will always remain as it's the type of the engine/engine management ... but if you're patient , all can be set/adjusted so that it drives really nice and smooth with still being right on the air/fuel mixture trough the rev range , not too lean neither too rich .

 

Tried doing the adjustments via ECU too (with variable potentiometers installed in the ECU) , adjusting/varying injectors pulse , and that along with the AFM spring tension adjustment made the most difference eliminating that jerky/shaky driving experience ... just ever so slightly increase of the injector pulse via ECU made it so much better/smoother .

But for doing the mixture adjustments , one must have WB gauge/meter plugged in the exhaust .. otherwise it's impossible to do the adjustments properly and precisely . 

 

After the mixture is adjusted , it's time for the ignition timing ... first i like to do some internal tweak on the dizzy springs (haven't tried on 1.6 , but on 1.9 it really works) then on to the adjusting the timing on the road "by the ear" ..

 

Afterwards , the end result is the Jetronic car which performs at idle and while driving much like it's Motronic variant - smooth and stable .

 

D

 

Edited March 14, 2020 by DamirGTI

[Ren89 0]

How do i check the resistance on the coolant sensor and ECU as that is something i have not done before and what should the correct resistance be?

If there is a problem with fuel pressure what would be causing it?

How do i check and adjust ignition timing?

 

 

[jackherer 543]

Unplug the coolant temperature sensor and measure the resistance across the two pins with a multimeter, the value you see will depend on temperature obviously, this is the table of values for a Bosch sensor:

 

 

If the value you measure is correct plug it back in (look at the plug first and check the pins are seated properly) then unplug the ECU inside the car, find the relevant pins for the coolant sensor then measure the value there to check it is the same as the sensor itself. I don't recall the pin numbers off the top of my head so you'll have to look it up in the Haynes.

 

If it's losing fuel pressure while not running the fault is a one way valve that is part of the fuel pump. If it's losing fuel pressure while running it could be the pressure regulator, the pump or possibly a leaking fuel pump hose inside the tank.