Lumpy running in an 8v Gti

[DS Andy]

I have a niggling problem with the 8v Gti which I'd appreciate any advice on.

It is fine under acceleration, and pulls really nice. Thanks to some replacement MI16 injectors it pulls great. But when you take your foot off the throttle it can get a bit juddery, and when you go back on the throttle it sometimes seems "slack". Also idling at junctions drop.

I did wonder if the throttle stop needs adjustment, but also wondered if this might be slack in the gearbox issue. Gearbox is quiet and very responsive under acceleration.

It's a good car - 90,000 miles approx with a new cambelt, clutch and pedal box and discs/pads last year. Made Fance and back, no probs, so long as you keep the leaky rad topped up!!!

[Mike E (uk)]

I would sort the leaky rad 1st, before the engine goes west.

These aluminium engines heat up really fast, and there is no header tank on the GTI, so you don't have much spare coolent if it leaks.

16V injectors in an 8V car--

Are the injectors different from the normal 8V fitting?
Or do they flow more?

( You don't get more power by adding more fuel.
More power is obtained by adding more air and a bit more fuel. )

If you are squirting too much fuel in, you would not notice under throttle, but idling may be poor.

So is it too rich at idle?

Also check the pressure regulator on the fuel bar-that can cause overfuelling too.

Is the ignition system OK, HT leads, HT cap+ arm.

Check the micro switch that operates at slack throttle is OK.

90,000 is not high for these engines if serviced well, the only thing that wears out are the valve guide oil seals.

hope this helps,

Mike

[DavidRutherford]

I was also thinking that this sounds like a rich-running engine. If the injectors have simply been fitted without re-mapping or any other alteration, the injector pulse will not be modified, and so if they are higher-flow injectors you will be running very rich.

What colour is the inside of the exhaust pipe at the tail? Black and sooty?

[docchevron]

/\ what they said. You'll be pumping through about 10% more petrol per power stroke.
So unless it's been mapped and it's moving more air then I'm afraid...PHAIL.

It's not white is it incidentally?
I've seen a white GTi floating about for a few weeks now, looks tidy too.

If so, it may have been you I blew away last Thursday week heading towards the M5 through Cribbs!

[Vanny]

Are 8v and 16v injectors not the same, with longer pulse duration on the valver? And isn't the mass flow rate controlled by the pressure regulator, and thus if the PR is not correct then this could be the likely culprit.

[DavidRutherford]

Are 8v and 16v injectors not the same,

I believe not.

isn't the mass flow rate controlled by the pressure regulator, and thus if the PR is not correct then this could be the likely culprit.

The function of the fuel rail pressure regulator is to ensure that the pressure differential across the injectors is constant, regardless of inlet manifold pressure. It means that the the relationship between injector pulse duration and injected volume should be the same regardless of inlet manifold pressure.

If the pressure regulator isn't connected, then you would indeed see a richer mixture at low manifold pressures (high inlet manifold vacuum) although I suspect it wouldn't be enough to have as much effect as is being reported. I've had a vacuum pipe to a fuel pressure regulator fail before, and the only effect I saw was increased fuel consumption. The engine still ran just fine.

[Vanny]

If the pressure regulator isn't connected, then you would indeed see a richer mixture at low manifold pressures (high inlet manifold vacuum) although I suspect it wouldn't be enough to have as much effect as is being reported..

Okay, but thats just an affset through the diaphram? The PR ultimately controls the return flow based on the pressure exerted onto the fuel system by the fuel pump? So if say the fuel pump was putting out double the pressure, then even at idle, or with the engine off then the PR would still maintain the 1.2bar of pressure in the fuel rail? And the vacuum pipe will adjust accordingly when running by way of an offset (and whoo hoo now i understand what the little fiddly pipe what i always forget to connect back up is for!).

[Vanny]

Specs for 16v injector;
Bosch part number: 0280150762
cc/min @3bar: 200
Electrical resistance: 15.9
Cap colour: yellow
Makes: PSA / Volvo
Engines: Mi16 / B230F
Models: BX, 309, 405, / 240, 740, 840

cant remember the specs for the 8v though, but thinking about it i imagine the 8v is single pintle while the valver will be 4(?) pintle?

[DS Andy]

Thanks guys, I really appreciate the input!

It's not running any richer than before, and prior to swopping the injectors it was really lumpy/pulsey under acceleration as well as lumpy at idle. The injectors I took out were a different part number, and black in colour, with cooked pintle caps and split o-rings etc. I do intend to recon them when I can find a kit like the one on the DIY pages!! And emissions are ok - it was MOT'd with these injectors two weeks ago.

I am inclined to check that micro switch at the weekend if it is warm and dry.

The leaky rad is a slow leak where the plastic header attaches to the rad - if anyone has a spare rad I'd gladly buy it. Until then it's kept topped up.

Doc - it is a white BX 1.9 gti reg E465 HDF with a Barack Obama bumper sticker. It is tidy - one front end scrape prior to purchase lets it down a bit, but it's a nice car for £200. My wife was up at Cribbs last week so yes, you probably saw it there.

[docchevron]

cant remember the specs for the 8v though, but thinking about it i imagine the 8v is single pintle while the valver will be 4(?) pintle?

Give the boy a prize!

Doc - it is a white BX 1.9 gti reg E465 HDF with a Barack Obama bumper sticker. It is tidy - one front end scrape prior to purchase lets it down a bit, but it's a nice car for £200. My wife was up at Cribbs last week so yes, you probably saw it there.

Ah ha! Didn't notice the sticker, or the scrape, from what I saw of it, it looked tidy, although at the time I was a tad late for work and giving it beans, it was rather a case of
"ooh, that looks nice, oh, it's gone"!
At the time my little black beast was bouncing off the rev limiter in third so time was short!