Not smooth on low rpm / floating rpm in neutral

[tadyzzz]

Hello everyone,

I have a classic mystery to solve. I have my GSX for nearly 15 years now, it's blue 2002, one of the first ones Bought it from my student loan, used, from France. A couple of years before I saw it in the magazine and it was love from the first sight. I lived a happy life, but 3 years or so ago it became not smooth under 3k - 4k. rpm. As I was riding mostly in the city it wasn't a big problem as acceleration from the red lights stayed quite smooth. Now I'm planning to take my GSX to the long trip and decided it's time to address this issue.

I noticed, that if higher rpm is maintained even on idle, it's floating and my guess is if I will fix this I'll fix not smooth riding too. I have a video on how it looks => https://www.youtube.com/watch?v=jQGdAMLsP-U. I first warmed the engine and then increased revs using an idle screw to eliminate throttle cables free play and human error as I'm not used to holding throttle steady

Or maybe GSX'es doesn't maintain steady rpm's on idle? I think there are probably less than 10 of these bikes in Lithuania so I can't check

I changed plugs, checked the resistance of both ignition coils, adjusted TPS and STPS, visually checked the condition of vacuum hoses,  tried running it with disconnected sensors which are not critical for the ECM (except TPS and STPS as everything was already connected back after adjustment  ). I checked all connectors I could find and they all look perfect, sprayed them with contact cleaner/restorer. The air filter looks like new.

Last year injectors were serviced, throttle synchronization was done and valves checked. It was performed by the repair shop and not by me, I started to think maybe I need to double-check even these.

I'm thinking maybe fuel pressure is not a constant and as a result amount of fuel changes all the time? Or it's too far fetched? 

I checked all (?) posts I could find in this forum with similar issues, found many, but no success yet.

Any ideas?

[Hooli]

Hello,

Under 3-4k (roughly) & at low throttle the bikes use the MAP (manifold air pressure sensor) to control the fuelling, above that the ECU calculates the airflow from the TPS & revs. That makes me think it might be the vacuum pipes or the MAP sensor. That's the sensor between the frame tubes above the engine that the vacuum pipes go to. It's the same part as the ambient air pressure sensor fitted to the RH side of the airbox, you could try swapping them and seeing if the problem changes.

Does it still happen with the MAP sensor unplugged?

Have you still got the PAIRs system fitted? that's got a vacuum controlled valve which could be faulty causing issues too. I'd try removing & blocking the vacuum line to that as a test.

[tadyzzz]

@Hooli, thank's for the reply. I have my PAIRS removed. I tried running it with MAP unplugged before, but after your suggestion, I still switched sensors to double-check, but no change.

Finally, I decided to check the fuel pump, as I have a pressure checking kit from eBay. I bought it maybe five years ago, but never used 

I found that when connected directly to the fuel tank pressure was ~2 BAR both on a running engine and before starting the engine. Then I checked debit and found that it pumped ~1 liter in 32 sec. Everything tested a couple of times, and I also borrowed another practically new battery from my garage mate. Same results.

So it's 1 bar lower and ~200ml less than specified in the manual.

I suspected that filter or mesh might be clogged, so disassembled the pump assembly and to make sure I checked the pump directly by attaching a hose to the output and submerging pump to the clean fuel. Tested it again using two batteries and found that it's precisely the same pressure and debit. Also, I noticed that a small hole near the fuel input of the pump was spraying fuel all the time - somewhere in this forum, I saw a scheme of the pump, and it should be a pressure reliever (not a regulator which is on the filter cartridge), so maybe it's defected.

I suspect ~30% lower output could affect running even in the low revs as injectors lack proper pressure, and engine suction is low at low rpm.

Now I'm in a hunt for any pump, as my trip is after nine days :O

Can't wait to see if this is the reason!

[Hooli]

I think I understand you & yes I think you're correct in how the outlets work.

I believe the filter & pump unit is the same in almost every Suzuki, it's just the metal bowl it fits into that changes. So the pump unit from a GSXR etc should fit into yours if that makes one easier to find.

You can still buy the high pressure filter new from Suzuki (the black bit) last time I did it was about £170, no idea how that'd workout in your money.

[tadyzzz]

I bought this one from the eBay => https://www.ebay.co.uk/itm/121629207664 . Seller will ship it using international express hopefully it will be here next week. Will see how it goes.

[KiwiCol]

Great, you've got a new pump, but no high pressure filter, which I suspect is where the issue lies & if you mount the old filter on this pump I'd bet the problem will still be there.   Grab a new filter whilst you're at it, or do the bypass mod & external filter.

Here's the link to that mod.  https://gsx1400owners.org/forum/index.php?topic=590.0

[grog]

Tady, def not enough pressure. Had same at work with VW car, while running that little hole was spraying fuel. Never found out why but new pump fixed it. Car pump and 14 pump prob the same pump, most EFI pumps similar. 14 stock pump is a Mitsubishi, forgot its model. I posted its model number on here or old org, no idea how to find that post. Will search again and post. 

[tadyzzz]

@KiwiCol I checked the pump itself separately, and it showed the same pressure and debit as measured with all the filters, so I think the filter is all good. I will wash it with carb cleaner and flush it in reverse with clean fuel, though. And I'm afraid to drill that hole couple of days before my trip, to be honest

@grog I have read all related threads, I saw these about compatible pumps from the cars and other models, they are here

I ordered a pump from eBay yesterday, and today I got it. It looks like a quality product much heavier than the original and all the fancy colors. From UK Northampton to LT Vilnius 2000km in 16 hours via DHL Next Day EU  It seems next day EU delivery exists 

I will put everything back and test it on Sunday.

[tadyzzz]

Everything assembled and tested. A new pump gives solid 3.4 bars of pressure, the engine starts easier, super smooth on low revs. I was riding around the city on 50 km/h, and I couldn't believe it's happening. It felt like I bought a new bike.

When someone encounters this topic in the search for an answer for rough on low revs, not smooth, jerky, etc. add fuel pressure/debit into the checklist. The pump should keep at least 3 bars and should be able to pump 1200ml in 30sec.

[Hooli]

Hurrah for fixing it! 

[grog]

Well done mate.  when you said you had low pressure, you were on track.

[Eric GSX1400K3]

Everything assembled and tested. A new pump gives solid 3.4 bars of pressure, the engine starts easier, super smooth on low revs. I was riding around the city on 50 km/h, and I couldn't believe it's happening. It felt like I bought a new bike.

When someone encounters this topic in the search for an answer for rough on low revs, not smooth, jerky, etc. add fuel pressure/debit into the checklist. The pump should keep at least 3 bars and should be able to pump 1200ml in 30sec.

Thanks for that, ive tried everything  else on my K3, apart from new chain and sprockets (next on the list) and its still slightly rough at low rpm.  Will start hunting for a new hp fuel pump.

[tadyzzz]

@Eric GSX1400K3 I'm really pleased with the pump from the link above, more expensive than AliExpress stuff but still cheap. Looks solid. Please let us know if the change of the pump helps!

[numbskull]

@Eric GSX1400K3 I'm really pleased with the pump from the link above, more expensive than AliExpress stuff but still cheap. Looks solid. Please let us know if the change of the pump helps!

Is it a straight swap, or are you taking some bits from the old pump to use?

[numbskull]

Thought it worth mentioning (my sample size of 1) that I also bought the above pump and fitted it last week. Chasing similar symptoms as the OP, and have also changed the injectors without curing it.

The pump failed after about 40 miles. Got home on the back of an RAC truck 

Took the "new" pump out. Several megohms across the terminals - completely dead. Put the original pump back in and went for a ride of 130 miles on Saturday.