GabeDiaZ27 Posted February 7, 2017 Share Posted February 7, 2017 Ok so recently I have been plagued with some rb problems. The engine will behave fine but when under boost the engine will bog/stutter kind of like a rev limiter or two step kind of deal but it wont rev much higher than maybe 4k-5k rpm. when there is no load on the engine (in neutral or clutch in) it will rev fine but not while in gear. So I did a lot of trouble shooting and found the cause of the problem, as the engine revs up the vacuum is INCREASING and causing my FPR to cut the fuel down to around 20 psi or so. From my understanding as the rpms rise the vacuum is supposed to DECREASE and go into positive pressure as the boost comes on but it seems that the opposite is happening. Im no master mechanic so I am just wondering what is causing this. could it be a timing chain slipped or something? Im honestly stumped and I cant find much info on this so I need some help or at least a way around this, Thanks in advance. Quote Link to comment Share on other sites More sharing options...
mtnickel Posted February 7, 2017 Share Posted February 7, 2017 Given the same load requirement, the higher the RPM the lower the vacuum will be. For instance, cruising at 50mph in 5th may require -10 to -5 inches mercury vacuum, but at in 2nd at 5500rpm, it'll probably be like -18 to -20 vacuum. You're assumptions of vacuum and revs are incorrect. What you're experiencing is that the engine can rev freely at very little load, but struggles to rev when high load is put on. This could be: 1) Bad coil packs. These engines are notorious for bad coils. When boost comes on, the mixture is much more dense and much harder for the spark to fire. bad coils mean the engine stumbles, pops, etc. 2) bad/fouled spark plugs. Same log as above 3) Are you running the stock ECU and the stock Airflow meter in roughly the same configuration it was used from the factory? changins to the intake flow path can affect the tune of the engine. 4) Vacuum/boost leak. Make sure you're hoses are all tight. Perform a boost leak test if you can. My first guess would be misfire from either 1 or 2 however. Quote Link to comment Share on other sites More sharing options...
Chickenman Posted February 7, 2017 Share Posted February 7, 2017 In addition to the above: Lack of fuel will also cause cause misfiring, lack of power under boost. Clogged fuel filter or a Fuel Pump going bad are just two of the causes. Boosted engines will really show any weakness in fuel area. Can be fine under light load, but will get worse with higher engine loads. An instrument panel Fuel Pressure gauge is as valuable as a Boost Gauge on a forced induction car. You can get remote units with reliable sending units that do not present the interior fire hazard as the old mechanical units. Problem is, weak ignition and weak fuel pressure/volume can cause almost identical symptoms. That is why you need to monitor Fuel Pressure in real time. Are you running a Boost Gauge? How much boost do you run and what do you have for a fuel pump etc? High Boost needs LOTS of fuel. Quote Link to comment Share on other sites More sharing options...
GabeDiaZ27 Posted February 7, 2017 Author Share Posted February 7, 2017 Its running stock boost pressure not anything more, and I have a walbro 255 pump and a nismo fpr. The problem isnt that there is a lack of fuel the problem is that as more vacuum is introduced to the fpr via manifold vacuum it lowers the amount of fuel pressure on the regulator. The engine should be loosing vacuum and go to 0 then into positive pressure as boost comes into play. The vacuum should be highest at idle then drop so more fuel is added as the rmps rise. I just dont know why my vaccume is low at idle then raising with the rpms Quote Link to comment Share on other sites More sharing options...
GabeDiaZ27 Posted February 7, 2017 Author Share Posted February 7, 2017 Sorry only saw that second post for a second. Ok yeah Ill check all the ignition stuff tonight and try to see if I can get further on this Quote Link to comment Share on other sites More sharing options...
mtnickel Posted February 8, 2017 Share Posted February 8, 2017 I'm case you're unaware, the fuel pressure follows the manifold pressure. The regulator is vacuum referenced. For example, (guess numbers) at idle at -10psi manifold pressure, your fuel pressure will be base pressure minus 10psi. Base pressure may be 43psi, so at idle it may be 33psi (again, these are very rough numbers for demonstration). When you ease into the throttle and hit 0psi, you will then see the base pressure of 43psi, and finally, as boost builds fuel pressure Will rise accordingly. The idea is that no matter what presssire is in the manifold, the pressure on the injector Nozzle is always the same. Makes fuel delivery predictable. Can you go full/moderate theottle at low rpm to get to 0psi (atmospheric pressure) and see that fuel pressure comes up accordingly? Do this under load in gear. Again, I reiterate it's the load and how much the throttle plate is open that dictate pressure, not just rpm. Quote Link to comment Share on other sites More sharing options...
mtnickel Posted February 8, 2017 Share Posted February 8, 2017 I'm case you're unaware, the fuel pressure follows the manifold pressure. The regulator is vacuum referenced. For example, (guess numbers) at idle at -10psi manifold pressure, your fuel pressure will be base pressure minus 10psi. Base pressure may be 43psi, so at idle it may be 33psi (again, these are very rough numbers for demonstration). When you ease into the throttle and hit 0psi, you will then see the base pressure of 43psi, and finally, as boost builds fuel pressure Will rise accordingly. The idea is that no matter what presssire is in the manifold, the pressure on the injector Nozzle is always the same. Makes fuel delivery predictable. Can you go full/moderate theottle at low rpm to get to 0psi (atmospheric pressure) and see that fuel pressure comes up accordingly? Do this under load in gear. Again, I reiterate it's the load and how much the throttle plate is open that dictate pressure, not just rpm. Quote Link to comment Share on other sites More sharing options...
Wizzurp Posted February 11, 2017 Share Posted February 11, 2017 Increasing rpm does not necessarily reduce vac. Redline the car and let off the gas, you will see higher vacuum than idle by a decent bit. If you actually have a fuel pressure gauge and you aren't guessing, check your fuel filter and see if it is packed full of shit. If you have a vac/boost leak you'd have higher pressure when In vac and lower fuel pressure than you should In boost. Regardless of that boost leak test it for sure, that'll cause all sorts of janky rich/lean conditions depending on where it is leaking. Quote Link to comment Share on other sites More sharing options...
trone Posted February 25, 2017 Share Posted February 25, 2017 Unplug the FPR and dampener, join the vacuum lines and see how it drives. Quote Link to comment Share on other sites More sharing options...
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