Engine running problem, doesn't want to rev above 3k

[mrmarcos 1]

Dear all,

 

I finally got round finishing off my restoration of my phase 1.5 1.6 gti 205. At the first test drive I found that the car was not running above 3k RPM under load. Under that it is sometimes running funny but it starts right up. I changed the rotor kap as there was a lot of oxidation in the plug therminals. New spark plugs fitted, rotor was nearly new when I started the restoration. 

 

The engine "stop" light is also showing when running. I checked the ignition module which seems ok because it works or it doesn't right? I suspect the TPS as I measured it and found out that there is no resistance when closed (I can hear it click) between any of the pins. Half way throug to wide open it gives me a constant reading of 0.9 ohm. In other words, from idle up to half it gives a resistance of infinity past half thorttle it goes to 0.9 and it stays there. 

What could this problem be? All plugs have been checked on corrosion and broken cables. 

 

Thanks in advance 

[jackherer 543]

Are any other lights on with the stop light?

 

Do all the appropriate warning lights work when the ignition is on and the engine is not running?

 

The ignition amp can cause this problem, where it attaches to the heatsink it needs to have some thermal transfer paste, if it is missing or incomplete the amp can overheat.

[mrmarcos 1]

HI Jack, that is quick! Thanks. The ignition module backplate was indeed corroded but I cleaned it up properly and used condutive gell when I screwed it back on. The other lights are showing up when not running yes. When not running there are not other lights showing. 

 

I forgot to mention that I checked fuel pressure. The pump, alle lines hard and flex, fuel filter are new. Checked the pressure and stays 3 bars at idle and through the revs. No drop at 3 bar. Checked when driving. 

[mrmarcos 1]

Correction: When running there is no other lights than "stop" showing. All fluids are fresh and topped up. 

[Thijs_Rallye 87 4 Cars]

Welkom hier

 

If I am not mistaken you shouldn't be measuring 0,9 ohm anywhere on the TPS. It should either be conducting fully, or not conducting at all which goes for the idle contact as well as the WOT contact.

 

Let me post this again

On 4/24/2018 at 3:25 AM, petert said:

The TPS (M251) is just a three position switch, thus has either zero ohms or is open circuit. There are three conditions - idle, part throttle and full throttle. I can't recall which way it goes, but at part throttle there should be no connection between pin 30 and pin 52 or 53. At idle, there should be a closed circuit between 30 and 52 or 53, then the opposite between 30 and 52/53 at full throttle. It's typical for the idle switch to fail and thus be open circuit. Open fitting a new TPS, many fail to set up idle correctly. You should be able to hear the switch open and close as you crack the throttle off idle. Checking at the ECU with a mutimeter between 30 and 52/53 is the only way to fully diagnose the problem. 

 

As for the stop light, the only things triggering the STOP light is the oil pressure warning switch (the small one) and the overheat sensor in the cylinder head.

 

If you disconnect the wire from the oil pressure switch and the light goes out it is probably the switch. The switch is a normally closed type which is opened by oil pressure. You can test the overheat sensor the same way. If the wire had chewn through the isolation somewhere the light will light up as well.

 

 

Thijs

 

Added the Jetronic ECU pinout. If you measure them from the ECU you can diagnose any wiring issues as well.

 

 

Edited June 21, 2018 by Thijs_Rallye
Edit: added additional Jetronic ECU pinout

[mrmarcos 1]

Thanks again Thijs. I will check it at the ecu. Disconnectiing the ecu put my meter on it and turn the throttle by hand in the engine bay. I spoke to an automotive electronic specialist and he told me it is more accurate to measure volts when the ecu is active as the ecu works this way. I will find out more tonight! 

[mrmarcos 1]

The oil pressure and cilinderhead are the only one? The coolant and brake fluid as well right? Wilfred de Ronde from 205 totaal told me the early engines can be read out with a special computer he has. The stoplight could be triggered by the ecu seeing a faulty switch or sensor. There are only a few as we know but he told me it is possible. 

[Anthony 1,003]

Low oil pressure and high coolant temperature are the only triggers for the STOP light.

 

Low coolant level or low brake fluid level will illuminate their respective lights on the dashboard, but won't illuminate the STOP light.

 

The ECU will not illuminate the STOP light in any circumstance - the only warning light the ECU will illuminate is the engine check light on 1.9 Motronic based models.

[Thijs_Rallye 87 4 Cars]

1 hour ago, mrmarcos said:

Wilfred de Ronde from 205 totaal told me the early engines can be read out with a special computer he has. 

 

As far as I know there is no diagnostics present in Jetronic ECU's.
 

For checking the TPS either will do fine, Volts or Ohms. For measuring Volts you will need to partly disassembly the ECU connector, I'd start with Ohms personally because it is less work.

Edited June 21, 2018 by Thijs_Rallye

[mrmarcos 1]

Unscrewing the 3 M6 bolts holding the ecu and pull the plug. Ohm reading on the pins will work fine. I have long cables for my multimeter so can rotate the throttle while reading the ecu. You need a differance in Ohm to read a difference in voltage anyway. 

Regarding the STOP light it is quite funny. When the engine is stone cold the oil pressure is more than find as well as the coolant is fine. Maybe I hit/ pinched a wire while putting the car togheter. Will check the instrument cluster. Will also check if the STOP light goes away buy pulling the respective plugs of the sensors (oil pressure and coolant temp). 

 

Thanks so much guys. Will update tonight with more info. 

[Thijs_Rallye 87 4 Cars]

There are three coolant senders, one for the ECU, one for the gauge and one overheat switch which is the one that triggers the STOP and overheat light. This is the one you should be looking for.

 

 

[mrmarcos 1]

Thanks! Didn't knew that one. Can be that it is broken as the coolant one gives me a good reading. 

[Anthony 1,003]

More likely that the wire is damaged and is shorting against the engine block or bodywork than the switch itself being faulty IMO.  Easy to unplug and rule out the switch - it's the taller of the two on the back of the thermostat housing.

 

That won't be behind the running issue causing it not to rev beyond 3k rpm under load.  I have seen both AFM and ignition amp failures cause this - a faulty ignition amp tends to feel more like a rev-limiter with a abrupt, violent cut, whereas the AFM tends to go very flat and have no power as the engine is starved of fuel.

 

The TPS can be unplugged for testing purposes - the engine will feel a bit flat and lack power at full throttle and the fuel cut-off on over-run won't work, but otherwise the car will drive fine.

[mrmarcos 1]

It feels as a rev limiter. You feel the compression of the engine as there is no bang anymore. AFM I can measure as well so will check that now together with the TPS. I think I will order a ignition amp anyway for the cost of around 30 euro's it can never damage anything. 

 

I can check the spark as wel with a stroboscope if it just cuts out when I come above 3k RPM. 

Edited June 21, 2018 by mrmarcos

[mrmarcos 1]

Stop light was a fautly wire. Fixed now. So one problem still too go! First tps then afm. 

[mrmarcos 1]

TPS is functioning correctly. Now afm

[mrmarcos 1]

AFM was the problem. Thanks for all info guys! What corrosion can do to electrics...