1.3MPI fast tick-over

[johnh510]

We have an 1998 1.3MPI Felicia which has recently become troublesome:

when warming up the tick-over would randomly change speed, anything from 1000 to 3000 rpm.

While trying to sort this out I cleaned around the butterfly valve as it was gummed up, but that didn't cure the problem.

I then measured the CTS and it was the cause of the variations.

New CTS, warm up fine, but..

The engine speed stays at 3000ish revs during a gear change, and it doesn't really want to slow down, but if you stay in a high gear until the revs are below 1500 it will then tick over at about 1000 (tends to hunt slightly).

You can also get the speed down by blipping the throttle.

As far as I can see the throttle butterfly shuts correctly, and the microswitch for throttle closed is working too.

I suspect that the sticking high idle was caused by me cleaning the butterfly valve, but I'm blowed if I can see, or measure what's wrong.

Anyone been here before??

[Smubble]

mines changes when i first start it up.. sometimes its at 750 rpm.. other times its 1250rpm.. and sometimes its a 1k

dont know what the cause is.. checked the butterfly valve as you said... was fine

going to try the spark plugs as they were gettin changed this weekend anyway and see if that helps

[TeflonTom]

the throttle body also has a bypass passage which controls the idle speed by regulating the amount of air flowing in when the throttle plate is closed, this is done by a small motor which kind of opens and shuts the bypass very rapidly, sometimes they get sluged up with crud or they break completely.

or it could be a vacuum leak after the throttle body (ie manifold gasket, brake servo hose, fuel pressure regulator hose)

hope this helps

[johnh510]

the throttle body also has a bypass passage which controls the idle speed by regulating the amount of air flowing in when the throttle plate is closed' date=' this is done by a small motor which kind of opens and shuts the bypass very rapidly, sometimes they get sluged up with crud or they break completely.

or it could be a vacuum leak after the throttle body (ie manifold gasket, brake servo hose, fuel pressure regulator hose)

hope this helps[/quote']

Thanks very much for that - I didn't know about the bypass.

The things I've had to work on previously have driven the butterfly from the opposite end to the throttle cable to regulate idle speed.

.

It's possible I've disturbed some crud with the aerosol cleaner I was using when cleaning the edges of the butterfly valve.

Regarding leaks - it's quite likely I've upset a piece of pipework when uncovering the bore of the throttle body.

I'll have a closer look when I can get access to the car during dry daylight hours.

[johnh510]

mines changes when i first start it up.. sometimes its at 750 rpm.. other times its 1250rpm.. and sometimes its a 1k

dont know what the cause is..

When the speed is too fast, try unplugging the connector from the coolant temperature sensor.

If the idle suddenly settles around the 900 mark you have a CTS fault.

You'll need to put the connector back, otherwise cold starting will be a problem:

AIUI, when a sensor fails open circuit the ECU substitutes warmed up engine values for the obviously wrong one.

This is OK to get you home, and isn't likely to damage anything, but you won't get the rich mixture needed for a cold start.

[fordfan]

It is not uncommon for inlet manifold gaskets to leak on this engine.

[johnh510]

the throttle body also has a bypass passage which controls the idle speed by regulating the amount of air flowing in when the throttle plate is closed' date=' this is done by a small motor which kind of opens and shuts the bypass very rapidly, sometimes they get sluged up with crud or they break completely.

[/quote']

Sadly this isn't true on the MPI 1.3 with Simos engine management.

The idle speed is adjusted by a stepper motor working directly on the butterfly valve.

Having patiently measured the voltages on all 7 of the leads* into the throttle body it seems that pin 3 carries throttle position voltage, and I can see it stick at 3.7 odd volts, instead of the 4.1 ish it returns to when the idle speed is correct.

The thottle mechanism isn't jammed, and I don't think there are any leaks - having sprayed a fairly volatile substance around all the edges of the inlet manifold while it was running without change in engine speed.

So it seems I've messed up my throttle position potentiometer in the throttle body by over enthusiatic use of a solvent cleaner when cleaning the butterfly valve.

It looks a horribly expensive thing to replace too - as they're the thick end of 300UKP at JHMB.

Does anyone know where I might get one cheaper?

=========================

* pinout as follows:

7(w) +12

6(bk) +12 or a bit less (probably stepper motor drive)

5(bl) 0V when throttle closed, +12 when open.

4(w/bn) 0V ground

3(b/y) +4.1volts throttle closed, goes down as throttle opens.

2(r/y) stabilised +5

1(y) +4.1 ?

I also measured the MAP/"thrust" and air temp, as I wondered about that too:

(w/b) 0v ground

(bk/bl) +0.7 volts (air temp with engine hot)

(r/y) stabilised +5

(bl) +1.8 volts at tick over, increases rapidly towards +4.5 on blipping the throttle -MAP.

=========================

I measured the voltages by placing a very fine piece of wire ( a single stand from some thin multicore jumper wire) inside the socket on one connector at a time and putting the plug and socket back together again.

The meter was an old analogue device, because I prefer the continuous indication, and the need for a little bit of current, rather than the apparent great accuracy of a digital meter which only samples infrequently and can give extraordinary readings from spikes with virtually no energy in them.

=========================

Disclaimer:

I am not a mechanic, and the voltages are what was measured on a throttle body which is (almost certainly) faulty. The interpretation of what those voltages represent is my own, and may not be correct.

The MAP and ATS voltages are probably correct.

[TeflonTom]

try this,

temporarily cut the yellow wire from the throttle body and splice it to the black/yellow wire and see if this fixes the problem. if it does then it probably means that the potentiometer is dirty.

[johnh510]

try this' date='

temporarily cut the yellow wire from the throttle body and splice it to the black/yellow wire and see if this fixes the problem. if it does then it probably means that the potentiometer is dirty.[/quote']

I'm not feeling enthusiastic enough to cut wires.

I did have a bit more of a play, and discovered that 6 (bk) seems to be stepper motor control as the +12 has pulses which more or less correspond to idle speed changes.

As I can't seem to get to the bottom of it with forking out a shedload of cash for a "maybe.." fix ( about 145UKP plus VAT etc from here] ) so it's booked into the Skoda garage on 6/6/6 :eek:

I'm a bit worried what's going to happen.

=================

UPDATE

=================

Back from the garage - "probably the throttle body, unless it's the ECU."

So that's 500 UKP for the first shot and 700+ UKP for the second.

I'll have to see what BBA can do, as the car is barely worth the Skoda price of throttle body, never mind the ECU too.

[majorwedgy]

hey John, what happened with your car? I have developed the EXACT same problem as you in the exact same way (being a bit trigger hapy with the carb cleaner) What was the problem and cure?

Cheers

Ant

[johnh510]

It took a little while to sort, but the reconditioned throttle body fixed it, as I mentioned in the other thread.

The Skoda dealer couldn't diagnose the fault, despite having had the car for a day.

Their best guess was either the ECU for £700 or the throttle body for £500, "I'd start with the cheapest.."

Parts from BBA carry a two year warrantee, how long are you planning to keep the car?

I have no connection with BBA, I'm just letting you know it was the cheapest for an expensive part with a decent guarantee, in the same way I've mentioned JHMButtco in the past as being an alternative to the dealers.

Forgot to mention that there are two throttle bodies, the Siemens for the 1.6, the Pierburg for the 1.3.

The £145 I mentioned above is for the 1.6. It's £175 for the 1.3.

Both prices plus VAT and carriage.

[majorwedgy]

Hey guys,

Just found an amazing site (providing i receive the bit as promised tomoro). Just purchased a throttle body with 30day warranty for

[johnh510]

Hey guys,

Just found an amazing site (providing i receive the bit as promised tomoro). Just purchased a throttle body with 30day warranty for