Surge in throttle then to idle, leads to almost stall (with video)

[Nheels]

Hi everyone, if you have a quick moment, view this video:

Motor was warmed, then quickly throttling up then back down to idle leads to this behaviour shown in the video. Couple of days ago it stalled and was a bugger to get back working. I've installed new caps, rotor, plugs, fuel filter, rebuilt fuel pump.

Anyone recognize this behaviour as a symptom of something going on or am I just being overly paranoid!?

 

[WalleyeSniper]

Hi everyone, if you have a quick moment, view this video:

Motor was warmed, then quickly throttling up then back down to idle leads to this behaviour shown in the video. Couple of days ago it stalled and was a bugger to get back working. I've installed new caps, rotor, plugs, fuel filter, rebuilt fuel pump.

Anyone recognize this behaviour as a symptom of something going on or am I just being overly paranoid!?

Fuel leaking or air leaking into the combustion process somewhere. Usually on carbed engines this happens when fuel slips by the welch plugs in the bottom of the carb. Fuel injection it happens when fuel injectors are faulty and sporadically allowing too much fuel to spritz in here and there.

What you're usually seeing is:
- all running normal, then
- fuel comes in too heavy (causing a surge)
- fuel KEEPS coming in heavy (causing almost a flood issue and hesitation)
- then it clears itself up and runs smooth again until it over-fuels again

Which engine, year, and type is this? Is there an ECM attached? That will help with narrowing down exactly what's going on by us being able to tell exactly which model and era the engine is from (Full injection vs Carb vs EFI, ECM vs no computer, etc etc).

 

[Nheels]

Fuel leaking or air leaking into the combustion process somewhere. Usually on carbed engines this happens when fuel slips by the welch plugs in the bottom of the carb. Fuel injection it happens when fuel injectors are faulty and sporadically allowing too much fuel to spritz in here and there.

What you're usually seeing is:
- all running normal, then
- fuel comes in too heavy (causing a surge)
- fuel KEEPS coming in heavy (causing almost a flood issue and hesitation)
- then it clears itself up and runs smooth again until it over-fuels again

Which engine, year, and type is this? Is there an ECM attached? That will help with narrowing down exactly what's going on by us being able to tell exactly which model and era the engine is from (Full injection vs Carb vs EFI, ECM vs no computer, etc etc).

Hello, it's a 2008 5.7gi-300-j. The surge in rpm is me throttling up then back to idle very quickly.

Getting the injectors cleaner and serviced was on the list, you think this almost stall is because of a dirty injector?

 

[WalleyeSniper]

Hello, it's a 2008 5.7gi-300-j. The surge in rpm is me throttling up then back to idle very quickly.

Getting the injectors cleaner and serviced was on the list, you think this almost stall is because of a dirty injector?

OOOOoohhhh... Lolol. I thought it was surging on it's own! Haha, sorry, must've misread.

Yeah, dirty injector can cause that for sure. When you're pulling back on the throttle, the ECM sends an instant signal to pull back on fuel. That hard pull back on fuel is just a little bit of reverb from the engine assimilating to the instant choke amount of fuel, especially if there's no load on the engine.

However, it could also be an air leak (bad seal somewhere inbetween the fuel line fittings, air slipping by a faulty gasket on the intake or around the injectors, etc). The ECM and fuel delivery is only as good as the vacuum circuits they rely on.

I'd take starter fluid and spray it around those various parts of the engine while it's running and look for the engine to surge when doing so (spraying around injectors, around fuel fittings, Fuel lines, intake manifold gasket edges, etc) to ensure there's no air leaking in when cutting back on the fuel. If the engine surges or bogs at all when you spray in any certain little fitting, gasket, or injector area, then that's your problem where too much air is making it into the system causing the fuel to lean out.

Given everything you've replaced, it's either getting an abundance of fuel and not enough air OR an abundance of air and not enough fuel (in that moment when it stumbles and recovers).

- Air leak somewhere, allowing the fuel to lean out too much.
- Vapor lock (a fuel line too close to a hot part of the engine, and the heat bubbles up the fuel in that area creating a "vapor lock" tgat would stall you out and cause hell trying t get it back started. We it really hot out the day it died on you?)
- Bad/Clogged injector(s)
- Leaky injector
- Bad ECM (doubtful unless it's been waterlogged).

 

[Nheels]

OOOOoohhhh... Lolol. I thought it was surging on it's own! Haha, sorry, must've misread.

Yeah, dirty injector can cause that for sure. When you're pulling back on the throttle, the ECM sends an instant signal to pull back on fuel. That hard pull back on fuel is just a little bit of reverb from the engine assimilating to the instant choke amount of fuel, especially if there's no load on the engine.

However, it could also be an air leak (bad seal somewhere inbetween the fuel line fittings, air slipping by a faulty gasket on the intake or around the injectors, etc). The ECM and fuel delivery is only as good as the vacuum circuits they rely on.

I'd take starter fluid and spray it around those various parts of the engine while it's running and look for the engine to surge when doing so (spraying around injectors, around fuel fittings, Fuel lines, intake manifold gasket edges, etc) to ensure there's no air leaking in when cutting back on the fuel. If the engine surges or bogs at all when you spray in any certain little fitting, gasket, or injector area, then that's your problem where too much air is making it into the system causing the fuel to lean out.

Given everything you've replaced, it's either getting an abundance of fuel and not enough air OR an abundance of air and not enough fuel (in that moment when it stumbles and recovers).

- Air leak somewhere, allowing the fuel to lean out too much.
- Vapor lock (a fuel line too close to a hot part of the engine, and the heat bubbles up the fuel in that area creating a "vapor lock" tgat would stall you out and cause hell trying t get it back started. We it really hot out the day it died on you?)
- Bad/Clogged injector(s)
- Leaky injector
- Bad ECM (doubtful unless it's been waterlogged).

Thanks so much.. I'm a bit intimidated by the spraying of starter fluid around, but I'll make the injectors my next project!

If there was an air issue, would a fuel pressure test on the rail determine that as well? Maybe a fuel regulator as well? I actually have two new Bosch ones to throw in.

 

[WalleyeSniper]

Thanks so much.. I'm a bit intimidated by the spraying of starter fluid around, but I'll make the injectors my next project!

If there was an air issue, would a fuel pressure test on the rail determine that as well? Maybe a fuel regulator as well? I actually have two new Bosch ones to throw in.

The starter fluid won't combust and blow up like you'd think. You just kinda use common sense and give little squirts here and there. It heats off a warm engine near instantly.

Fuel regulators are always a good item to have in place, because if pressure is too high in spurts it can leave too much fuel unburned in the cylinder (causing a big issue) and can also overpower the restrictions in the system and flood out the system momentarily lie you're saying.

To check overall fuel pressure while it's operating, you can put an inline "T" on your main line(s) going to the system with a pressure guage on it. Monitor the readings when throttling quick and see if your fuel supply spikes or cuts out completely when you cut back on the throttle (this will show if your pump pressure is too high. Rebuilt doesn't always equate to correct pressure of course).

Then, if that checks out, check it at the rails inline, and see if one or the other is dipping out (indicating an injector allowing fuel to bleed through or leak somewhere inline coming to the rail) or if it spikes and has a tad delay (showing a clogged injector).

Just know, it's more than likely too much fuel or too much air getting in when isn't supposed to. Whether that's a sensors doing (like the faulty Mercruiser style ambient air intake sensors) or faulty/clogged injector, or vacuum/air leak somewhere inline from fuel pump to the injectors.

 

[Nheels]

The starter fluid won't combust and blow up like you'd think. You just kinda use common sense and give little squirts here and there. It heats off a warm engine near instantly.

Fuel regulators are always a good item to have in place, because if pressure is too high in spurts it can leave too much fuel unburned in the cylinder (causing a big issue) and can also overpower the restrictions in the system and flood out the system momentarily lie you're saying.

To check overall fuel pressure while it's operating, you can put an inline "T" on your main line(s) going to the system with a pressure guage on it. Monitor the readings when throttling quick and see if your fuel supply spikes or cuts out completely when you cut back on the throttle (this will show if your pump pressure is too high. Rebuilt doesn't always equate to correct pressure of course).

Then, if that checks out, check it at the rails inline, and see if one or the other is dipping out (indicating an injector allowing fuel to bleed through or leak somewhere inline coming to the rail) or if it spikes and has a tad delay (showing a clogged injector).

Just know, it's more than likely too much fuel or too much air getting in when isn't supposed to. Whether that's a sensors doing (like the faulty Mercruiser style ambient air intake sensors) or faulty/clogged injector, or vacuum/air leak somewhere inline from fuel pump to the injectors.

Good stuff. Thanks for the advice and replies. Take care.

 

[QBhoy]

Hi. Clean or replace the iac filter sponge. It’s not going to be injectors

 

[Nheels]

Hi. Clean or replace the iac filter sponge. It’s not going to be injectors

Ah, that actually makes sense from what I've been reading. Now I don't see a filter sponge on the schematic. When I look up the Volvo Penta PN 3843750 it doesn't show up.

From what I see it is an easy replacement, but any advice you can offer would be appreciated. Thanks!

 

[QBhoy]

Likely behind the throttle body or part of it