Brava won't idle

[Stuart]

The engine on my 1.6 died at a roundabout a week back. I got it started but it wouldn't idle so I ordered a new idle valve from Fiat. Fitted it tonight but still no joy. The car misfires when it's first started and won't idle on it's own even when warm. It doesn't run too badly once it's on the move but idling at 3000rpm gets you noticed at the lights

I measured all the sensors with a DVM probing through the connector at the ECU and they all give the kind of readings you'd expect apart from the Lambda sensor - heater wires read low resistance (OK I think) and sense wires read open circuit (not sure about this).

Any ideas? Is the Fiat diagnostic likely to find the problem?

Stuart

 

[Tom]

The lambda sensor is a diode, it will read open circuit. It actually generates a voltage when its hot.

Do you get an injection warning light on the dash? If so its probably a dead injector. Take it to Fiat and they can read the fault code.

Otherwise it could very well be coil or plug leads. The lambda has sod all to do with idling, they don't get hot enough to work properly.

If it died while driving its probably not idle valve, however it might be worth taking it off again and making sure the hole it fits into is nice and clean.

If there is no injection warning light, try these in this order:
Check coil resistances
New plugs
New plug leads
I don't think its throttle position sensor but could be the manifold pressure sensor.

Best of luck!

 

[stuart]

Re: Re: Brava won't idle

Tks for the ideas. I guessed the Lambda wouldn't measure easily.

I haven't had any warnings on the dash so far. The coil measures fine, the plugs are new and I get an OK spark when I hitch the leads up to a spare set of plugs.

I'll give the Fiat diagnostic a try otherwise I could be swapping bit of engine management for ever.

What I really need is a friendly scrapper with a good 1600 engine management system.....

Tks for the ideas.

Stuart

 

[andy1881]

manifold pressure sensor

One of these went completely on mine but it still idled fine so I dont think its that, it just seems to cause sudden losses of throttle pressure whilst driving

andy

 

[Tom]

Re: Re: Re: Brava won't idle

If there is no light Fiat can't do anything. They are mostly thick monkeys and if there is no fault code there is nothing wrong.

Seeing a 'good' spark does not mean it is a good spark. I know from experience that Fiats are very picky about having VERY good sparks. Try a new set of leads anyway.

There are some other things to try before you go ripping the ECU out. I have a good feeling it may be map sensor as a friends bike had a duff one and would not idle either. If you have access to a 'scope it would be worthwhile watching the output from some of the sensors while the car runs.

MAP
Throttle pos
Crank
Cam

 

[Stuart]

Re: Re: Re: Re: Brava won't idle

Yeah, I have a scope here. Not too sure how I can get access to the pins in the connectors - maybe I can hook a thin wire in the connector? Do you know what amplitude the cam/crank sensor pulses should be? I guess the throttle sensor is just a DC voltage varying? I'll try check them out tomorrow, see what I find.

Tks again

Stuart

 

[stuart]

Re: manifold pressure sensor

Engine is now idling (just) but very lumpy. I just found that nothing happens if I disconnect the cam sensor. Does that sound right? I wouldn't think the engine would run at all with a duff cam sensor?

Stuart

 

[Tom]

Re: Re: manifold pressure sensor

Does the injection light come on when you do that?

The cam sensor is only required for the fuel side of things. I would have expected it to die as it would not be able to sync the injectors.

Weird!

 

[Tom]

Re: Re: Re: Re: Re: Brava won't idle

You should see an AC waveform from the cam and cran sensors, it should pulse up, gap, pulse down, gap....

Dunno about amplitude, IIRC the sensors I used for my 3rd year project ECU were about 1v pk/pk.

Have you measured the resistance of all the injectors? Large connector on the right hand side of the engine, tucked into the big bend in the inlet manifold.

 

[stuart]

Re: Re: Re: manifold pressure sensor

The cam sensor seems OK. I get a clean 5V pulse train, 3 short then 1 long. If I disconnect the cam sensor no warning lights come on and the engine note doesn't change. There is also good continuity back to the ECU from the cam sensor output.

One other question - if one of the injectors is blocked will a warning light come on? I'm gonna try some injector cleaner today anyway. Also try to bodge in another set of HT leads as i don't want to spend ££££££ getting nowhere and the leads measure OK on a DVM.

Stuart

 

[Tom]

Re: Re: Re: Re: manifold pressure sensor

The ECU can only detect a failed injector, they usually go open circuit.

I would have expected the ECU to notice the cam sensor was missing. Weird!!

If you leave it idling for a while and then take the plugs out is one of them a different colour to the rest?

 

[Stuart]

Re: Re: Re: Re: Re: manifold pressure sensor

No, the injectors all measure OK. I also bodged in another set of HT leads and no difference.

Took it out for a drive tonight...goes like a train above 2000 revs but jumps like crazy at low throttle settings and won't idle at all (just dies).

I figured out the cam sensor thing. If you disconnect the sensor and start the engine the light comes on. If you disconnect it while the engine is running the light stays off. Seems like its only read when the engine first starts?

If I get time tomorrow I'll check crank sensor, throttle sensor and chuck in some injector cleaner for good measure.

Stuart

 

[Tom]

Re: Re: Re: Re: Re: Re: manifold pressure sensor

Hmm... very odd.

If it was a blocked catalyst it would not go very well at all.

Try the throttle pot and MAP sensor. A friends motorbike would not idle due to a nackered MAP sensor. The ECU thought the inlet manifold was +ve pressure at idle and got confused and cut out.

 

[stuart]

Re: Re: Re: Re: Re: Re: Re: manifold pressure sensor

OK, the throttle sensor seems OK. I guess its just a pot? Terminals A-B measure 1k1 and doesn't change when rotated. A-C goes from 865R (closed) goes up to 2k then back to 1k78 (full throttle). B-C goes from 1k9 (closed) to 2k then back to 700R (full throttle). Not sure why it goes up then down but otherwise it looks OK (not noisy or anything).

The MAP sensor is 700R A-B and about 43k A-C and B-C. Sensor is airtight but reading doesn't change at all with vaccuum on the inlet pipe. Is it inductive or something?

Stu

 

[stuart]

Re: Re: Re: Re: Re: Re: Re: Re: manifold pressure sensor

Just put the MAP back in and tried it - running like a dog but tonight it ticks over OK, just lumpy.

If I leave the MAP connected to the ECU and take off the vacuum pipe (tape over hole on manifold) the engine runs the same. What difference should the MAP make when it is connected or disconnected?

Stu

 

[Tom]

Re: Re: Re: Re: Re: Re: Re: Re: Re: manifold pressure sensor

Hmm.. I think your map sensor is stuffed. I'm pretty sure they are simply resistive and it should change.

It should make a big difference if you disconnect it. At low throttle you would have alot of manifold vacumn. At higher revs you have less so that could be why its running ok at speed.

Time for a trip to Fiat I think!

 

[stuart]

Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: manifold pressure sensor

Got a new MAP sensor on Saturday. I also routine checked the cam-belt and found the belt had jumped a tooth on the exhaust cam. I guess the guy who replaced my head gasket in Jan didn't tension it up enough? I reset the cam timing and lined up the mark on the tensioner again.

Anyway the engine sounds really sweet now but I'm stuck with two small problems still....

Since it's been running again the airbag light comes on most of the time. Don't know why this would be?

There is a noise like a rough bearing from the cambelt area at between 1k-2k revs. It's OK at high revs or at tickover. I reckon it sounds like it's coming from the belt tensioner but it was definitely replaced just a few months ago, together with the fixed roller (got the receipts). Do these ever fail again this quick or am I paranoid?

Stu

 

[johnnalino]

Do you have a stethoscope?

Like a mechanics steth? listen to the tensioner area and if there is a bad noise, return it to the place that swapped it. Tensioners are temperamental if overtorqued. I remember when the stalk that the tensioner sat on in my 1.4 snapped off. Which was a laugh. I had only overtorqued very slightly. It is not unreasonable to expect garage work to last at least 3 months, regardless of a warranty being offered or not.
It is easy and slightly cliched to criticise Italian build quality, but I suspect the low quality of FIAT's british technicians has perpetuated this myth.

 

[Tom]

Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: manifold pressure sensor

Sounds like you were lucky there. A misstimed cam is usually the last thing on the list..

Time to get the listening stick out and find the source of the noise. Its a bit odd that it goes away at higher revs. If something has been over/under tensioned then its always possible a bearing has gone.

At least you are getting to the end of your quest.

 

[stuart]

Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: manifold pressure sensor

Yeah, I checked the cam last just on the off-chance while I was waiting for the MAP to arrive. Don't know what fixed the problem but I'd have given up long ago without help from this list.

I think the noise has been there for a while I just noticed it more lately because I spend most of my life with my head in the bonnet!

Listening around with my ear on a long screwdriver pressed to bits of the engine the tensioner does seem to be making the noise. I guess I'll get a new one. This has gotta be the last job (except the airbag....).

Stu