Engine wants to die under extended load
06-30-2018, 09:02 PM
#1
Staging Lane
Thread Starter
Engine wants to die under extended load Hi,
My 95 Z28 M6 has developed a problem where it starts losing power while under extended load and then wants to die until you give it a bit of time to "rest". The engine is a stock rebuild with 35k km and 241k km on the body. Opti is a NOS AC Delco unit with about 10k on it. MAP sensor is also new. Plugs and wires were replaced 35k ago during the rebuild.
At first it seemed to happen randomly but now I kind sort of tell when it will act up. The engine needs to be under load for a while, such as when on the highway in 6th gear for a while, when accelerating hard onto the highway, or I've even managed to intentionally cause it by driving around in 3rd while keeping the RPMs high. I can feel it getting a bit rough and sluggish, and then if you give it throttle it wants to die and a few times its made a noise that sounds like a backfire. It will keep doing this until coasting for a bit and then the engine will make power again.
I've got several datalogs, some are normal and some show the problem when it's happening. I think it's a fuel issue as the BLMs rise and in some cases will hit 160, even under steady throttle. I bought a fuel pressure gauge but the hose is only like 12" long, so I can't stick it on the windshield while driving to monitor the fuel pressure. I'm not sure if there's a cheap extension hose available. At idle the pressure is 35 PSI and it stays around that when I rev the engine. I'm going to try checking it later when the engine cools with the regulator vacuum disconnected to see if it raises.
In the log JJ2, around sample 950 you can see where the BLM starts acting weird. I know in one of the logs I saw where the BLM went from normal and then jumped to 160 while the TPS stayed the same, which is one of the times it acted up. Strangely the O2 volts seem to show it as being rich when this happens.
I'd appreciate any insights, as right now I don't know where to start looking other than finding a way to monitor the fuel pressure while driving.
My 95 Z28 M6 has developed a problem where it starts losing power while under extended load and then wants to die until you give it a bit of time to "rest". The engine is a stock rebuild with 35k km and 241k km on the body. Opti is a NOS AC Delco unit with about 10k on it. MAP sensor is also new. Plugs and wires were replaced 35k ago during the rebuild.
At first it seemed to happen randomly but now I kind sort of tell when it will act up. The engine needs to be under load for a while, such as when on the highway in 6th gear for a while, when accelerating hard onto the highway, or I've even managed to intentionally cause it by driving around in 3rd while keeping the RPMs high. I can feel it getting a bit rough and sluggish, and then if you give it throttle it wants to die and a few times its made a noise that sounds like a backfire. It will keep doing this until coasting for a bit and then the engine will make power again.
I've got several datalogs, some are normal and some show the problem when it's happening. I think it's a fuel issue as the BLMs rise and in some cases will hit 160, even under steady throttle. I bought a fuel pressure gauge but the hose is only like 12" long, so I can't stick it on the windshield while driving to monitor the fuel pressure. I'm not sure if there's a cheap extension hose available. At idle the pressure is 35 PSI and it stays around that when I rev the engine. I'm going to try checking it later when the engine cools with the regulator vacuum disconnected to see if it raises.
In the log JJ2, around sample 950 you can see where the BLM starts acting weird. I know in one of the logs I saw where the BLM went from normal and then jumped to 160 while the TPS stayed the same, which is one of the times it acted up. Strangely the O2 volts seem to show it as being rich when this happens.
I'd appreciate any insights, as right now I don't know where to start looking other than finding a way to monitor the fuel pressure while driving.
07-01-2018, 04:12 AM
#2
I took my auto parts store pressure gauge to a radiator shop and had them extend the hose to something like 3 feet so I could tape it on the windshield right in front of my face. This certainly sounds like a fuel issue. I've had pumps give me grief plenty of times especially when it's hot out. i finally installed a digital fuel pressure gauge on my a-pillar so there's no more surprises. keeping an eye on pressure when your symptoms act up will help rule out the pump.
I suppose it could also be your O2 acting up. do you have the ability to run the car in open loop? if it still does this with the O2's disabled that will rule out the O2 sensors.
I suppose it could also be your O2 acting up. do you have the ability to run the car in open loop? if it still does this with the O2's disabled that will rule out the O2 sensors.
07-01-2018, 03:09 PM
#3
you can just un-plug the 02's and the car will stay in open loop to see if the problem persists
sounds more like a fuel issue.
while the 12" hose is short...the Schrader valve is rear of motor. Isn't there enough length to run it up on the cowl to even see it through windshield?
I have had the Hypertech FPG for 15 years. It has a hose long enough to easily tape it to windshield
viewing the gauge during a SUSTAINED WOT pull will show if FP drops.
also if you have a scan tool and STFT start dumping fuel under this condition it would indicate low fuel pressure because the car is leaning out
sounds more like a fuel issue.
while the 12" hose is short...the Schrader valve is rear of motor. Isn't there enough length to run it up on the cowl to even see it through windshield?
I have had the Hypertech FPG for 15 years. It has a hose long enough to easily tape it to windshield
viewing the gauge during a SUSTAINED WOT pull will show if FP drops.
also if you have a scan tool and STFT start dumping fuel under this condition it would indicate low fuel pressure because the car is leaning out
07-11-2018, 08:37 PM
#4
Staging Lane
Thread Starter
I managed to connect a guage and it turns out that I definitely have a fuel pressure problem. My car thinks it's got a carb, as the fuel pressure drops to 5 psi at times while babying it!
It seems to stay low for a while and then eventual it will jump up in small amounts and settle in the low 30's. The car is undriveable in its current condition. Key on, engine off it will sit at 41.
Now I'm not sure how to figure out if the problem is with the fuel pump or the pressure regulator. The pressure never drops to 0 and it never stalls, so it doesn't seem like the pump is stopping.
Is it possible for the pump to get tired/overheat and slow down, or for the regulator to have a weak spring or dirt that holds it open?
It seems to stay low for a while and then eventual it will jump up in small amounts and settle in the low 30's. The car is undriveable in its current condition. Key on, engine off it will sit at 41.
Now I'm not sure how to figure out if the problem is with the fuel pump or the pressure regulator. The pressure never drops to 0 and it never stalls, so it doesn't seem like the pump is stopping.
Is it possible for the pump to get tired/overheat and slow down, or for the regulator to have a weak spring or dirt that holds it open?
Last edited by chronos; 07-11-2018 at 08:44 PM.
07-19-2018, 12:48 PM
#5
Staging Lane
Thread Starter
I replaced the FPR and that didn't help, so it seems that the problem is in the tank.
Will I only need a replacement pump, or should I get the whole bucket assembly as well? It seems to hold pressure overnight, so I'm guessing that all the tubing is intact.
Will I only need a replacement pump, or should I get the whole bucket assembly as well? It seems to hold pressure overnight, so I'm guessing that all the tubing is intact.
07-19-2018, 04:51 PM
#7
TECH Enthusiast
Cannot stress the part about checking the in-tank wiring enough! If you are not getting a minimum of 12v back there then you will likely be replacing fuel pumps frequently. Don't be surprised to find it might be somewhere around 10v with the old crappy wiring used in these cars.
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07-27-2018, 03:32 PM
#8
Staging Lane
Thread Starter
I monitored the wiring at the body connector and the voltage was between 12.3 - 12.8v while driving. It rose to 13.0v when the pressure was at 0 psi.
I am in the process of replacing the pump with a new AC Delco unit. Inside the tank the wiring looked good and everything looks good. The tank itself has no rust or sediment and still looks new.
When I cut open the old pump, the only thing I found is that one of the three plastic tabs that locates the bottom of the pump against the the bottom of the bucket was broken off. I don't know if this has any effect or not. I suppose it could allow the pump inlet to move and cause a leak between the inlet and the sock. However, I thought the bucket is normally kept full of fuel and this wouldn't matter.
Could the 13v reading with 0 psi be due to the pump spinning with no load because it's sucking in air?
I am in the process of replacing the pump with a new AC Delco unit. Inside the tank the wiring looked good and everything looks good. The tank itself has no rust or sediment and still looks new.
When I cut open the old pump, the only thing I found is that one of the three plastic tabs that locates the bottom of the pump against the the bottom of the bucket was broken off. I don't know if this has any effect or not. I suppose it could allow the pump inlet to move and cause a leak between the inlet and the sock. However, I thought the bucket is normally kept full of fuel and this wouldn't matter.
Could the 13v reading with 0 psi be due to the pump spinning with no load because it's sucking in air?
08-14-2018, 10:56 PM
#9
Staging Lane
Thread Starter
Finally got it all back together and now it drives great! I decided to order and install a Magnaflow catback at the same time. 
I also bought a Pacesetter ORY, gaskets, ARP bolts, and O2 extensions to go with my Pacesetter LTs, so hopefully I'll get those installed next.
My butt dyno might need recalibrating after driving a Rav4 for 2 months, but the Camaro feels a lot stronger and more responsive that I remember it being for a long time.
With the old pump, the pressure didn't consistently rise to ~43 psi, it was usually around 35-36, so I wonder if it was running a bit lean at times and that was making it feel sluggish.
I also bought a Pacesetter ORY, gaskets, ARP bolts, and O2 extensions to go with my Pacesetter LTs, so hopefully I'll get those installed next.
My butt dyno might need recalibrating after driving a Rav4 for 2 months, but the Camaro feels a lot stronger and more responsive that I remember it being for a long time.
With the old pump, the pressure didn't consistently rise to ~43 psi, it was usually around 35-36, so I wonder if it was running a bit lean at times and that was making it feel sluggish.
