Technical Persistant idle problem Uno 1.1 ies

[Demolite]

Hi,

I've been struggling with the idle of my 1991 Uno 1.1 ies. Once the engine temps hit about 70-80 on the gauge, the problems appear. At idle the engine starts surging at a pace of once every 2 seconds. RPMs decrease and possible misfire occurs (this can't be seen clearly in the cylinder burn time measurement I made). Also sometimes when dropping to idle from revs, the revs go very low and might stall. Runs great otherwise.

I have tested/changed almost everything: plugs, plug wires, wiring, grounds, distributor cap and rotor, igniter module, vacuum advance, vacuum hoses, carbon canister valve, head gasget, injector, throttle body, fuel lines, fuel pump, fuel filter... Ecu has been reset each time.

Also, after starting the alternator lamp is always lit until I rev the engine a little. This is non-dependant on engine temp. When starting with engine hot, surges are very light until alternator starts charging (engine revved a bit). I haven't tested a new alternator, but have changed the regulator.

Closest I got to solving it was when I changed the throttle body, it ran great for half an hour. I tried adjusting the TPS, with no improvement. Electrical parts have been tested with a multimeter and oscilloscope. Oxygen sensor values loop:

RICH-LEAN-RICH-LEAN-LEAN(surge)RICH-LEAN-RICH-LEAN-LEAN(surge)....

Cause or symptom?

The surging gets worse in cold weather, when I changed the throttle body and noticed improvement I did it in a warm garage.


Any ideas?

 

[Demolite]

I solved the problem, it was in the coolant sensor circuit. The sensors resistance checked out ok, so it had to be in the wiring or ecu voltage divider circuit. That's why a new sensor didn't help. The voltage at ecu terminal 2 (coolant temperature) was under 0.3 volts when warm. Resistant short to ground or bad pull-up resistor.

Problem was that ecu thought engine was over 100 C all the time when warm. The engines whole temperature scale was shifted about 30 C upward, causing lean running. Surges were caused by lean misfire as ecu was trying to stick to its +100 C fuel map.

Quick fixed it by connecting a trimpot between pin 8 (5v output) and pin 2 (coolant temp input) on the ecu connector. Bosch manual says 0.9 V at the coolant pin is 95 C. I found that 0.7-1.1 work fine with warm engine. Needless to say cold starting was greatly improved. Set value of 2.8k worked for me.

For a permanent fix, I will open the ecu and change the divider resistors.

I hope this helps someone, I had this problem for over a year.

 

[a73uk]

Thats excellent information, thank you for letting us know & well done on tracking it down!

 

[nunesaf]

I solved the problem, it was in the coolant sensor circuit. The sensors resistance checked out ok, so it had to be in the wiring or ecu voltage divider circuit. That's why a new sensor didn't help. The voltage at ecu terminal 2 (coolant temperature) was under 0.3 volts when warm. Resistant short to ground or bad pull-up resistor.

Problem was that ecu thought engine was over 100 C all the time when warm. The engines whole temperature scale was shifted about 30 C upward, causing lean running. Surges were caused by lean misfire as ecu was trying to stick to its +100 C fuel map.

Quick fixed it by connecting a trimpot between pin 8 (5v output) and pin 2 (coolant temp input) on the ecu connector. Bosch manual says 0.9 V at the coolant pin is 95 C. I found that 0.7-1.1 work fine with warm engine. Needless to say cold starting was greatly improved. Set value of 2.8k worked for me.

For a permanent fix, I will open the ecu and change the divider resistors.

I hope this helps someone, I had this problem for over a year.

well done and thanks for sharing! Very nice of you mate. Which Bosh manual are you referring to! i have been trying to find the specs of the uno bosch monotronic for quite some time with no luck.

 

[Demolite]

I found the information in the Polish Mono-Jetronic manual posted on this forum. There is also a Bosch yellow-book manual which is more detailed in some situations.

However the information might have been wrong. After opening the ecu, I found nothing wrong with the voltage divider circuit. It consists of a 2.6 kOhm pull-up and a 36.5kOhm pull-down resistor. The calculated voltage for coolant pin at 90 C (= sensor ~200 Ohm) is actually 0.4 V. There seems to be contradicting information in some of the manuals.

Anyway, I still now know the reason for not running right, lean misfire. While the symptoms were corrected with modifying the coolant signal, the problem still lurks elsewhere.