82 replies to this topic

[Robbie693]

Hello everyone,

 

I have been using Sprocket's excellent guides for years in sorting out problems, also the posts by many users such as FlyingScott, DaveRob to name but two but I have come up against this problem for which I find I need to actually make a post about!

 

This is a bit rambling but please bear with me - the questions are at the end!

 

I have had fast idle problems in the past in the past, usually cured by changing the vac hoses and it started again recently but seemed to improve after doing some servicing work, -  new steady bushes top and bottom, new air filter, dizzy cap, rotor arm, spark plugs water pump, water pump pulley and alternator. Also adjusted the valve clearances, flushed the cooling system and fitted a new oil pressure sender. When the alternator failed it did so very suddenly, I was driving home and switching on the lights I noticed the dash illumination to be very dim, this got worse as I continued to the point that the headlights were like candles and the wipers barely made it across the screen unless I turned the mains off and drove on sidelights. 

 

 

All was fine after this apart from having to adjust the fan belt a couple of times as it stretched and started to slip. The new problems started up whilst the belt was slipping on the way home last week.

 

Now the idling problem has become worse, revs hanging like the throttle is stuck. Driving the car it revs up to 1800 rpm as the clutch is pushed in, slowly dropping down to 1200 before pulling away again. Lifting he throttle with my foot does nothing and I have pushed on the linkage to the throttle body to confirm the cable isn’t sticking.

 

It Idles at 1200 rpm from cold as you would expect,  but continues through to warm up and stays there until throttle is revved whereupon it will hang around the 1500 - 1800rpm range varying  a bit, eventually dropping back to 1200ish. It is not fluctuating rapidly as some posts have mentioned, more just a slight  variance of about 50 rpm around the1200 rpm mark or wherever I am holding the revs with the throttle.

 

During this time the stepper motor is pushing the revs - if you open the throttle the stepper plunger will catch up with the throttle and hold it at 1500 rpm or so for a few seconds as the throttle returns to idle, then it motors back in and the revs settle back to 1200. If I just crack open the throttle the stepper will actually increase the revs to 1500 or so before settling back

 

At first. If the lights - main beam and spots -  were switched on he revs dropped to 900, fairly stable, and then rose again when lights switched off. Battery voltage was low possibly due to the slipping fan belt, I have charged it now and the lights only bring he idle revs down a bit. The ‘sticking’ is the same however.

 

 

Investigations so far:

 

Fuel trap needed changing - there was some fuel in the lines between the manifold and the trap but also in the one between the trap and the ECU. There was also a bit if brown residue in the ECU outlet when I took it out to have a look so I dried to out on a radiator as suggested.

 

Vac hoses themselves were fine but I changed them anyway though it made no difference.

 

Temp gauge reads mid way on the white tick as it always has done, occasionally raises up to the black line in traffic.

 

I have adjusted the throttle cable and set it by indexing the stepper motor as in the manual. The stepper motor does the usual thing of indexing as you switch off too.

 

Plugs are new (100 miles or so) and still fairly clean but there is a small amount of carbon build up around the edge so it appears to be running a bit rich.

 

Fitted a new air inlet temp sensor but this made no difference either

 

 

Rambling over, now the questions!

 

Now I have done all the easy to get to stuff I am turning my attention to the Coolant temp sensor, Lambda sensor or stepper motor...

 

 

From my description does it seem that the stepper is operating correctly?

 

With the temp gauge behaving correctly does this mean the CTS is ok?

 

As I have no hesitation, misfiring or any other symptoms of poor running can I assume the Lambda sensor is ok?

 

Does any of this give an indication of what to try first?

 

Cheers

 

Robbie

 

 

 

 

[cian]

I think before you spend any more money is to get a computer on it, you can easily spend hundreds on an mpi/spi and get nowhere

[FlyingScot]

You haven’t said explicitly but looks like SPi?

Double check vacuum lines and fuel trap to ECU

Then check TPS using either voltmeter or multimeter method 

 

FS

[genpop]

if you have the chance, measure the fuel presure. In any case change the fuel filter, possibly the fuel pump too!

[MacGyver]

Not sure it'll help, but my friends mpi did this sometimes. He had to switch off and back on again every so often.
It seemed the throttle cable was slack but just not enough.
The weight of the pedal made it slightly open and reved a little.
Maybe try to slacken it a little 'too much' just to rule that out?

Ben

[Robbie693]

Ah yes - apologies for leaving out the crucial information! It's a 1996 SPi 

 

Point taken on the computer, I have a few ideas about where I may be able to get it tested but I thought I'd make plans for DIY'ing it if this doesn't pan out.

 

Thanks for all the test suggestions, I will get on to it. I have also been reading about the importance of the valve clearances being absolutely correct and it's made me paranoid that maybe one or two have slipped/moved or whatever so I intend to re-check these.

 

I can't find the post about the TPS check at the moment but as I recall it involved testing the resistance between the middle pin and one of the outer two and noting any spikes is that correct?

 

Robbie

[genpop]

if you have the chance, measure the fuel presure. In any case change the fuel filter, possibly the fuel pump too!

Forget about this, i didn`t read your description totally.

But measure the voltage at the generator when the car is running.If the voltage is higher than 14,8v there, your generator has a fault and the higher voltage may cause the stepper working wrong.

[Robbie693]

Alternator output seems fine - 14v at the battery. The battery does get low sometimes usually because the car does short journeys with the lights on at this time of year, but it always starts ok.

 

I have had a go at testing the TPS but I'm not sure I am doing it right - I read on here somewhere that you can put the multimeter probes on the pins of the TPS and check for continuity and note the resistance changing as you open the throttle (and any spikes/dropouts). Well I put the meter on all combinations of the 3 pins and got nothing - no continuity, no beep, no resistance, if I pushed the throttle of not.

 

Doest this mean something is amiss or am I doing it incorrectly?

[cian]

Have you got continuity from the plug to the ecu? What does it run like with the plug disconnected? Any worse?

[genpop]

14V at the battery is too high, you should measure at the generator!!!!

The TPS is a resistor only.

Out of a manual:"The TPS sensor receives a 5 volt supply from the ECM on a YP wire and is supplied with an earth on a KB wire. The sensor then provides a signal which is proportional to throttle disk position on a YG wire to the ECM."

[Robbie693]

Alternator output is 14v, when I say 14v at the battery I meant when the engine is running/charging.

 

I didn't check the TPS for volts as I couldn't get at the pins in the back of the plug without hacking the insulation about and I'd ran out of time anyway.

 

I haven't tried disconnecting the TPS plug, will give it a try and see what happens.

[pete l]

Why is 14v too high ?

 

I have a Rover 75 that puts out 14.6 volts at the battery, over on the 75 forum I was told this is fine !!!!!

 

Pete.

[genpop]

"Why is 14v too high ?"

 

I just wanted to express that the charging voltage should not be extremely above 14V (i.e. greater than 15 or 16V). His problems started with the failure of the generator, so it could be that too high a voltage would affect the work of the ECM.In his case I would have the ECM reset once, and of course a log file would also be very useful.

[Quinlan minor]

I had the same problem, seven or eight years back, on my MPi.

When stuck in traffic, the revs would rise, and rise: 2,000 RPM+ on occasion! I went through all the panic diagnoses: cracked throttle body and the like. Mike, at Croydon Minis, suggested a new throttle cable. Problem solved and it's never returned.

[FlyingScot]

Alternator output seems fine - 14v at the battery. The battery does get low sometimes usually because the car does short journeys with the lights on at this time of year, but it always starts ok.

 

I have had a go at testing the TPS but I'm not sure I am doing it right - I read on here somewhere that you can put the multimeter probes on the pins of the TPS and check for continuity and note the resistance changing as you open the throttle (and any spikes/dropouts). Well I put the meter on all combinations of the 3 pins and got nothing - no continuity, no beep, no resistance, if I pushed the throttle of not.

 

Doest this mean something is amiss or am I doing it incorrectly?

You can test the TPS one of two ways 1) reading volts and connected up - ignition on and back probe the middle and outer pins. Moving the throttle should show 0.4 to 5 volt swing 2) pull the plug to the TPS measure resistance between middle pin and outer. You are looking for smooth transition without dropping out (falling to open circuit circuit)