erratic low revs (hunting)

[guest4461]

I (my wife) has a ’54 plate sports jazz, when you start it from cold , tick over and throttle response are as should be, if left to warm up you then try to ease the throttle peddle down to increase revs slightly , the revs go down from 8 or 9 hundred rpm to 4 or 5 hundred rpm then jumps up and down 500 to 2500rpm (hunting) without moving the throttle peddle! If you accelerate more or take your foot of completely the fault goes. Had it been a carb I would of expected a pilot jet (smallest one ) problem but being an old git I haven’t a clue on injection systems. No noticeable problems through the rest of the rev range .
Any ideas ?
Martyn

[guest4461]

Would a petrol additive (injector cleaner) help with my problem? if so which one is recommended ?

[guest4078]

Sounds like an annoying problem, bad luck.
You could try a good shot of redex or another fuel system cleaner and/or use a couple of tanks of posh petrol. But it does sound like it might need a bit more than that. You could put a new air filter in, a 2 minute job and a fiver from a motor factor. You could try cleaning the throttle body with carb cleaner. Check the induction system for leaks. The egr valve is probably grimy by now, but it doesn't sound like that would cause your symptoms. Is the exhaust gone? If the cat has disintegrated that might cause problems or damage a sensor? Throttle issue?
Best of luck.

[guest4461]

thanks for that, since my original post i have cleaned the egr and looked at the air filter, its filthy so I have a new one coming tomorrow but tested the car on the front drive with the air filter removed and the fault was still there so the filter is not acting a a choke. This car is only used 3 nights a week 5 miles to and 5 from work , once a week for shopping 10 miles each way, though in late Sept we went 360 miles each way to Scotland and back with another 300 miles whilst up there, just over 1000 miles in a week and the problem didn't change, you just get use to driving it with the problem (good revs when pulling away)

I will check the induction system for leaks and get some redex injector cleaner and some carb cleaner tomorrow and try both (one at a time so if any do the trick I will know which one worked!)
 
There are no warning lights on the dash but will look for fault codes with a mates code reader.

Thanks again,
Martyn

[GSX600FMAN]

Check for vacuum tube leaks, look for perished tubes. Also pull off any easily accessible connectors and reconnect just in case there is a poor connection. I had similar years ago on different car and it was cracked/leaking vacuum tube to throttle body etc.

[guest4461]

I have now tried all of the above with no luck , though the fuel additive will take time to show any changes but i am going on a 160 mile round trip on Saturday , mostly on dual carriageways, see if that helps!

there is what I believe to be a vacuum pipe that goes to a blue electrical unit on top of the throttle body, I have no idea what this unit is for but it may be faulty ? any help on this unit, it has 2 wires , the plug is in good condition and clean.

 This problem only occurs once the engine is up to working temperature.

Thanks for all the ideas so far 

[guest4461]

I have searched for "Honda jazz vacuum" and finally identified this unit as "PETROL VACUUM PUMP VALVE 1362002650265"

What does this actually do and is there a way to test it before I try and replace it and would it be the cause of my problem?    so many questions

Picture attached below  (I'm getting cleaver for an old git!)



[attachment deleted by admin]

[guest869]

Try looking at the butterfly and look for black gunge in the throttle body. Clean the black off. It cured my similar issue.

What happened on mine is the black stuff was causing the butterfly to stick a bit. The ecu raised the revs a bit but the butterfly sticks so it opens the butterfly a bit more until it jumps to high revs. This is too high so it then closes the butterfly.

[guest4461]

Thanks, I have sprayed some carb cleaner in but need to strip it back a bit more and do a thorough job to eliminate that as the problem. They don't make cars easy to work on nowadays, just another way for them to take money of you. My hobby is motorbikes from the 40's and 50's and I can do anything on them at the roadside!

Thanks again,
Martyn

[culzean]

My hobby is motorbikes from the 40's and 50's and I can do anything on them at the roadside!

Modern cars are chalk and cheese compared with the old ones with dodgy ignition and carburetors - they were easy to work on,  but they needed to be !! I can remember abysmal build quality and constant fixing, as well as MPG not too good either.

one of my workmates who hadn't ridden one for years was asking what sort of motorbike he should get,  simple really --- if you want to have a bike as a hobby and spend lots of time in your garage tinkering with it get a British bike pre 1990, but if you want to spend time riding it and not have too many problems get a Jap bike of any age.  I have no fond memories of any British bike I had,  with their terrible reliability and dodgy handling - much prefer not getting my hands dirty these days.

[guest4461]

Once set up right and regularly maintained Brit bikes can be OK .I have ridden my '59 650 cc BSA , 2 up and loaded with camping gear, all over Europe and even shipped it out to Australia for 6 weeks, carried sensible spares and tools and sorted it at road side when needed. But I do agree Japanese bikes were/are pretty bullet proof. When my son started on bikes his new Honda 125 exhaust lasted 2 years before it had a hole in it (thin and cheap) my BSA system has lasted 20 years , yes it has rusted but still solid and usable and less expensive to replace

Back to topic

I went 80 miles to my sons place (first trip after putting Redex Injector cleaner in tank) and by the time I had got there the fault had gone 
80 miles back home and still OK, in Sept I went to Scotland and back with no improvement so it must  be the Redex Injector cleaner.

By the way , I looked in Tesco's and it was £4 , B+Q £3 .... Halfords £4.99 same stuff, same size 

Thanks all for your suggestions, I have learnt a lot from this , hope others have to

Martyn

[culzean]

I am a great believer in the power of Redex and similar fuel system / injector cleaners - bought my motorbike back to health (very hard to start and would not tick-over) after a winter layup - car seems to run better after a dose as well 

[guest4078]

Glad you got it fixed. You can't beat a drop of redex. I would also use molyslip at next oil change to make sure all is nice and smooth inside the engine and to keep the top end well lubricated.

[guest4871]

Good result. Well done.