VOLVO 5.7 GSI Stumbles at 2800 RPM

[h2oelectric]

2000 Volvo 5.7 TBI Here is what I know and some of what I have done.
- Boat is a heavy 26 '
- boat worked great after the winter
- problem started while enjoying the boat
- replaced fuel filter
- checked screen filter before the lift pump (was clean)
- checked screen filter before high preasure pump (very slight debris) and still stumbles and
backfires at 2800 RPM, does not stall
-checked high presure with a confirmed good tester, tested at just under 30 PSI (quite sure it should be between 32 and 36)
possibly this is the problem ?
- checked the pressure on the VST and never saw any more then 4 PSI (not sure what it is supposed to be ? )
- both pumps are fairly quite with no excessive heat.
- any assistance or help would be greatly apprieciated.

 

[skywidget]

Re: VOLVO 5.7 GSI Stumbles at 2800 RPM

I have a 5.7 GSI that was running great then suddenly started loosing power at ~2500 -2700 RPM. At least this was the first observed behavior. After a few test runs and lots of help from guys on this forum, I started reading about the S.L.O.W mode in the diagnostic manual. Some folks refer to it as Guardian mode. Mine is a 1999 PowerQuest and that manual refers to it as S.L.O.W mode. Essentially the ECM (Computer) monitors both Engine Coolant Temperature and Oil Pressure. If either falls out of range the ECM will shutdown half of the injectors or in my case 1 of 2 Throttle body injectors to reduce power protecting the engine without shutting it down. I'm assuming it's to allow you hobble back to shore. The other half of this is it will continue to shut off half the injectors as long as the engine remains above 1200 RPM. At 1200 RPM it will re-enable all injectors.

So here's the behavior:
1) At throttle up, the engine will begin to build power/rpm.
2) At ~2500 RPM it will shut down half the injectors causing the engine to reduce power/rpm.
3) At 1200 RPM it will enable the previously disabled injectors and power/rpm will begin to climb again.
4) If the throttle, is reduced at any time during this sequence and power/rpm remains below 2500 RPM,
The ECM will not disable the injectors and operation will "appear" to be normal.

*If you throttle up and when you reach the 2500 stumble point and you leave the throttle in place, the ECM will cause the engine to cycle between 2500 and 1200 RPM.

To test if this is the case:
1) Slowly throttle up until it reaches the ~2500 RPM until it "stumbles". (Leave the throttle in position, do NOT reduce)
2) The engine should begin to slow and at 1200 it should catch (injectors return) and begin climbing back to ~2500 RPM.
3) Continue to leave throttle in the position where you left it at Step 1.
4) It should stumble again at ~2500 RPM and cycle as mentioned above for as long as you don't move the throttle.

If this turns out to be your symptoms, then it's pretty easy to confirm. After reading the Diagnostic manual cover to cover I ran into a couple sentences that said to "disconnect both the plugs from the ECT sensor and the Oil Pressure sensor. If the behavior disappears you have a bad sensor. Then you just have to determine which one by connecting the plugs back one at a time. Mine turned out to be the oil pressure sensor. It's a little more tricky to get to it than the ECT. The ECT is right up front.

Again, not sure if this is exactly your trouble but I started my thread very similar as you.

 

[h2oelectric]

Re: VOLVO 5.7 GSI Stumbles at 2800 RPM

thanks skiwidget. unfortunatly the guardian mode is not the problem. tried the test and found that if we leave the throttle at the point where it stumbles at 2800 RPM, the motor runs at about 1200 RPM and will not cycle back up until the throttle is backed off. I did try disconnecting the oil pressure and coolant temp for a brief trial with no change.
Very open to suggestions, should anyone have any.

 

[Tail_Gunner]

Re: VOLVO 5.7 GSI Stumbles at 2800 RPM

Take a neon light and watch your injector's spray pattern..nice conical shape no drip's.

 

[h2oelectric]

Re: VOLVO 5.7 GSI Stumbles at 2800 RPM

We have a nice conical shape until the stumbles, then not so nice. once the throttle is decreased the spray returnes to nice.

 

[Tail_Gunner]

Re: VOLVO 5.7 GSI Stumbles at 2800 RPM

Describe not so nice umm does it still spray??? does it spit.. does it pulsate at at a even rate take notice of each injector

 

[h2oelectric]

Re: VOLVO 5.7 GSI Stumbles at 2800 RPM

all good guys, found the second oil pressure sender, with a dead short,
replaced the sender, now life is good beyond 2500 RPM
Thanks for all your help. (this is a great forum)

 

[Tail_Gunner]

Re: VOLVO 5.7 GSI Stumbles at 2800 RPM

Apperently one injector cut off...skywidget was dead on.

 

[h2oelectric]

Re: VOLVO 5.7 GSI Stumbles at 2800 RPM

Absolutely Skiwidget had it right. however in this case the engine did not cycle as described in the test. When we left the throttle at 2500 where it stumbled the motor would go to 1200 and stay there. Thanks again guys.