'Stuck' revving problem - V6 Petrol

[onehillside]

So, our beloved V6 petrol has intermittently developed a fault whereby she won't go above 1,000/1,200 revs. She idles fine but when the accelerator pedal is pressed it's like there's a blockage in the exhaust or something (if that makes any sense); there's almost a thud noise and the revs won't go higher. She runs ok at idle and gear changes up (ie. I can limp along at 10mph on the flat, 2mph uphill or a heady 30mph downhill, with accompanying changes up or down gear).

The exhaust is fine - I've been underneath her and there's no issue. There's no nasty smoke, rattles, thuds or holes evident.

The frustrating thing is that this problem is intermittent - a 100+ mile round trip up to Ikea and back, filled up with furniture : fine. Two days later - get a mile to the village to get the morning papers and from whilst getting there is ok, on starting the engine on return - the issue re-appears. Limp home, park her up and the next day she's fine.


I'd hazard a guess at air filter / oxygen sensor / MAF and loose wiring perhaps but I would greatly appreciate any input before I took her off to the garage...

[ELZE]

Hi

Just a long shot but when did you last fill it up with petrol? I ask because I had a similar experience a few years back and the end result was simply a tank full of dirty petrol which cost me a fortune to sort out.

Of course the service station denied it but I went to the trading standards but that's another story.

Good luck

[onehillside]

hmmm - it's got a nearly full tank and I usually fill up from a mix of forecourts (normally without the level dropping below half) and not heard of dirty fuel locally but it's a possibility. I'll add that to the list of things to check out - thanks!

[ELZE]

It's strange why a perfectly running Bongo should go from a healthy running machine to a choking spluttering state.
Was it a supermarket fill up? If so was it named with an A this one in particular are well documented for selling dodgy fuel.

Just Google it and be amazed ??

[Simon Jones]

I would suggest changing the fuel filter as a good starting point. When I took mine out, there was a lot black crud that came out. It under the van, drivers side, just in front of the fuel tank, so a bit of a pain to access.






After that, you'll probably need to start investigating the ignition system. Another thing to try is to loosen the petrol filler cap to see if there is a problem with the breather system.

[onehillside]

well - changed the air and fuel filter today - the latter wasn't exactly the easiest to get to - lying on your back in the cold trying to get both hands up and round fuel pipes whilst trying to avoid petrol dripping into your nose / hair etc Anyway - all done now...

..and short answer is that it doesn't appear to have solved the issue - she's fine on start-up and will rev freely but then is still refusing to get above 1000 revs, there's a 'knock' or thump noise occasionally but I can put my foot right down and still nothing, she just refuses to rev past it. will sound like she's trying but no dice.

I don't think it's air supply - unplugging the sensor on the air filter box doesn't sort it or let her rev any more freely and that was the best guess of a mechanic friend, so i'm guessing it's still possible fuel related though he's pretty sure it's going to be electrical especially as the frustrating thing is that we had similar symptoms earlier in the year; she'd been sitting for a while outside and then when started down the road just wouldn't rev. So I just unplugged all the sensors on both the drives and passengers side, gave them a wipe down to remove any dampness, left her to run for 30 minutes and warm up and the issue seemed to have solved itself. Then she was fine for a couple of hundred miles and a month or so of intermittent use.

And now it's back and I'm jiggered as to what the issue is.


Is there a Workshop manual available for the V6 or a guide to which sensors are what on either side of the engine?

I'm not a complete novice when it comes to cars and would rather try and work through to find the cause myself if possible - last time we had an issue that had to go to the garage we ended up paying for a good many hours of investigation before they found out what the problem was. I'm more than happy to work around my 1993 MX-5 though there are 1001 different available manuals, online guides and books to help you - this site is a boon but it'd be useful to have the workshop guide if such a thing exists...

All advice gratefully received!

[Simon Jones]

Club shop sells manual that covers both petrol engines. It's worth ordering the chassis manual too as you get a bit of a discount. The engine manual covers diagnosistics and how to test the sensors.

http://www.igmaynard.co.uk/bongo/mercha ... tm#Manuals

If you haven't already done so, try disconnecting the lambda sensor on the exhaust and see if that makes any difference.

[onehillside]

ahhhh - hadn't seen those (for some reason the Bongo Fury is mysteriously and largely absent from Google search results) in the shop. I'd best stock up on reading matter then! I figure money spent in that regard is going to be an investment.

Thanks for the suggestion re: L.sensor - will double check tomorrow when it's light again.

[onehillside]

well - disconnected the lambda sensor this morning to check that's not the issue and no dice, problem still there.

So;

- Engine starts fine
- Revs freely (up to and above 3000 no problem) at cold / on startup.
- When warm / after a short while refuses to rev above 1000. Will just bounce revs back down, occasionally knocking.
- She changes up / down gear without issue - I'm currently limping along to the local school carpark at weekends to keep brake discs and pads from seizing etc.

I have;

- replaced air filter; no difference.
- replaced fuel filter; no difference.
- tried loosening fuel cap to see whether that indicates an issue / helps; no difference.
- checked with MAF sensor disconnected ; no difference.
- checked with lambda sensor disconnected; no difference.

It has a full tank of petrol bought from a reputable garage (not supermarket or independent) that we've used intermittently for years without issue.

Just bizarre. The symptoms presented themselves a while back for a short time and then resolved and were fine for a couple of long trips (50+ miles) and a bunch of short, 10 mile round trips. Then one day randomly re-presented themselves. Presumably the fact that she's fine at cold and the issue presents itself shortly afterwards / when slightly warmed up means something...

I'm ordering the manuals at the end of the week once payday comes around. I guess next step will be seeing what the diagnostics suggest / whether they point to anything.

[teenmal]

Hi onehillside,I don't know if its been mentioned or not but it might be worth while having the Crank Shaft Position Sensor look at.

Good luck.

[dunslair]

Has the mechanic checked the wiring loom to and from the ECU and the voltage in and out of the ECU? Our ECU (from 2l petrol) is currently off being tested and hopefully 'fixed' having experienced similar problems. We had already changed the MAF sensor and checked the other sensors following the fault codes. I hope to hear back from the garage over the next couple of days with the outcome, so I will let you know.

Cheers

David.

[onehillside]

at the moment it's just little me (with help from here ) trying to see if I can isolate where the issue lies. I'll be ordering the manuals later in the week to try and see whether I can diagnose issue from ECU code (and better understand what / where the various sensors etc sit on the Bongo);

£70 odd will be well spent - aside from the satisfaction of sorting it myself, given our local Bongo specialist charges quite a lot by the hour I might save myself some money too...

(I'm no mechanic but I'm reasonably confident around my MX5, aside from the differences in scale, it's not too big a jump to working my way around the Bongo)

[kennie0503]

My freda is doing exactly the same would love to know if there was ever an answer to this problem

[Simon Jones]

Could you give us a bit more background info such as when/how it started, what has been done so far, have all the normal service items such as plugs and filters been changed, as well as HT leads, distributor cap and rotor arm? Have you got LPG and if so, does the problem happen when running on gas?

[haydn callow]

Check out your ignition system.