Resolved! Jerky at slow speed

I've been experiencing a problem with slight surges/jerkiness in town traffic ever since rebuilding my engine.
I have not been happy with the IACV making a clacky racket at tickover. I know it's supposed to open/close to maintain 800rpm or so, but surely not several times a second!

Here is the list of my efforts to resolve it:-

Dismantled and cleaned the IACV - it was a bit sooty.

Checked for overtight K&N on the throttlebody intake.

Checked for intake manifold leaks.

Verified there was a small amount of slack in the throttle cable at rest.

Checked throttle butterfly and cable for smooth operation (lubed the cable, greased the pivots)

Cleaned every wiring connection in the engine bay with switch cleaner - especially sensors and injectors.

Tried injector cleaner in the fuel.

I ought to have know better - it was the water temperature sensor passing inaccurate results to the ECU.
Therefore the ECU was controlling the fuelling for a cool engine - causing overfueling - and probably why the MOT emissions test took several attempts to "pass" the idling tests.
The temp gauge was fine - under half way at all times, but that feeds from the black sensor under the brown one!.

Now I have no more slight surges/jerkiness in town traffic, improved throttle response and a quiet tickover.

So.... for those with MOT emission problems, try changing the brown sensor - only costs a tenner!

For those that aren't sure how to change the sensor here's my method:-

With a cold engine - raise the back of the hood and remove the engine inspection panel.

Locate the brown sensor at the top of the engine near to the exhaust manifold at the gearbox end. It is next to the hot hose outlet.

Disconnect the wiring plug from the sensor .
It is secured by a U shaped wire clip. Use a pair of thin pliers to hold the wire clip and pry one of the sides out with a small screwdriver. Use the pliers to withdraw the clip and the connector will pull apart.
Put the U clip back into the connector's grooves - the clip will automatically open and shut when securing the plug to the new sensor. The plug will only fit one way round.

Have the new sensor to hand and smear a little RTV gasket goo on the mating face where the thread starts.
Some sensors have a metal sealing washer like a bear bottle top - it's worth re-using on you new sensor if it's not already present. Be sure a smear of RTV is present both side of this washer - but don't have any on the threads.

Use a 19mm ring spanner to unscrew the old sensor - you will lose a very small amount of coolant when it comes out.
You might want to have some rags under to catch this.

Fit the new sensor - it's a plastic body so don't try to overtighten it. Tight is good enough.

Refit the wiring clip.

Add a small amount of coolant to the overflow tank - to replace what was lost.

Run the engine until warm - checking for water leaks.

Refit the engine cover etc

Job done.


Hope this helps somebody, Jeff

Excellent troubleshooting and how-to. :thumbsup:
Thanks Jeff!

Jeff,
Well done !

did you try to overcome probable connection problems at the plug to the sensor before you changed it ?

I ask cause I did the same change job several years ago at my F.
Checked the suspect old dismantled part later with hot water and a multimeter but couldn't find it faulty.

Problem came back half a year later. Finally the reason was a problem with the sensor wires at the ECU connector a my MGF.
So keep an eye on it.

you know your stuff jeff,impressed with your knowlegde and how you go about things.we have a finished sub frame with engine and gear box .I would like to invite you over wednesday after noon to see our work on refurbished cars.
the sensors are always going wrong on these cars,we are lucky because we can simply un bolt and try a different part and we can plug the car in and hope fully the computer will tell me what and where the fault lays.mind you they are basic engines so I feel I can talk to the car and she talks back.may be I spend to long round these cars.Let me know if you want to pop over

I'll have to try this as my car is a bit of a pig on tickover and will only pass emissions when up to full temp.

One thing that did improve the low speed traffic thing was fitting polybushes to the lower engine mount. Felt much better after that.

http://the-t-bar.com/en/forum/22-cjjs-guides/15329-torque-tamer-bushes-how-to-fit

Yes Dieter, I am sure the connectors were clean and tight on the duff sensor (and at the ECU end).

During rebuild everything was stripped, checked, refurbed or replaced. Each connector male and female was scraped, washed in switch cleaner and dried with air from a tin. Before refitting each had a shot of WD40 and any excess mopped up with a rag..

I had the ECU and it's panel off the car. Dismantled that to reveal all the relays and connectors - they all had the same treatment before refitting.

Jeff

its the brown connector ,also check the wires in the connector its self.the wires are thin and as time goes by the become bittle.Any way you shouldnt be driving slow

xad3888 wrote: .....and dried with air from a tin.....

I didn't know you could buy tinned air :coat:

Tinned air can be got from Maplins, PC World etc. It's very expensive though at £15 a tin.
Ebay is your friend - http://cgi.ebay.co.uk/Pack-3-Airdusters-400ml-Air-Duster-Computer-Cleaning-/260659261852
That's three 400ml tins supplied and posted for less than the price of one in the high street. Bargain.

Tinned air used to be propelled by CFC's which are a no no for the environment, but cheap. Now they use more expensive inflammable gas instead - go figure!

Jeff

petevick wrote:

xad3888 wrote: .....and dried with air from a tin.....

I didn't know you could buy tinned air :coat:

I've got a machine that traps all the air around it and puts it in a big tin. I then connect a hose and use it.

I have this same issue on my 2003 MG TF.
Do you have any photos on how to locate the sensor?

:welcome2: