Engine stumbling

[FFRSpec72]

On a very short coruse (1.5 mile) this weekend, with 2 180 turns, engibe stunbles comming out of these turns, like a fuel issue, otherwise no issues, the fuel pressure is consistent, any thoughts?

[FFRSpec72]

Replaced fuel filter, still had issues, engine would stumble (backfiire, misfire, buck,etc) on low RPMs after comming off of a 5500 RPM run going into a 180 hard turn, good fuel pressure, plenty of fule, no ussies at high RPM. Not sure what is going on. Maybe a bad plug wire. that is shorting?

[Brian Z]

What kind of spark plugs are in it? Could also be the TFI module on the distributor. Has it stalled out yet?

[Brian Z]

It actually sounds like a vacuum leak. That could cause a backfire and it would smooth out at high rpms.

[IEMRacing]

I had the same problem......
It was the coil.
hope this helps
John

[FFRSpec72]

What kind of spark plugs are in it? Could also be the TFI module on the distributor. Has it stalled out yet?

Autolite 3924, engine has not stalled out, idles fine, revs up fine, except for comming out of the corners after high rpm run, I can ease into it all is fine but when I go full throttle this is where it stumbles

[FFRSpec72]

It actually sounds like a vacuum leak. That could cause a backfire and it would smooth out at high rpms.

Only have 1 vacuum line, from fuel pressure regulator to manifold and that looks good no issues

[FFRSpec72]

I had the same problem......
It was the coil.
hope this helps
John

John, I have the orginal 93 coil so it may have given up, I will see if that is the issue, normal driving all is fine, so only way to test is back out on track. I did move coil when I installed power steering so maybe I did something then. So did your coil work fine under normal load?

[mhoward1]

Are both 180's the same direction?

[FFRSpec72]

Are both 180's the same direction?

nope opposite, basiclly any hard corner seemed to give me issue it seems

Also there is no engine check light on, will pull codes but I don't expect to find any with no check light

[IEMRacing]

John, I have the orginal 93 coil so it may have given up, I will see if that is the issue, normal driving all is fine, so only way to test is back out on track. I did move coil when I installed power steering so maybe I did something then. So did your coil work fine under normal load?

Yes, it was fine under normal loads.
Where I got it was coming out of T3 and T4 at laguna seca.

I went to the dyno with Brian Rebello and started with plugs, plug wires, distributor cap, rotor, and then the COIL..........

I replaced the msd coil with a Kragen coil and it was show-time........

Give it a try.

I hope this helps

John

[Scott McKay]

You super sure the fuel pressure is fine? It sure sounds like a fuel pickup issue. What was the fuel level in the tank?

[FFRSpec72]

You super sure the fuel pressure is fine? It sure sounds like a fuel pickup issue. What was the fuel level in the tank?

I usually run a 30 min race with about 6 gals, so I added 12 gal and still had the issue. So as far as I can tell there is no fuel pressure drop

[vnmsss]

Good advice here already......Certainly check the coil....Perhaps you can borrow one from someone just to assure that's not the issue.....I'd be looking at the fuel pressure as a very likely source, as you cannot assume that just because you have enough fuel in the tank that it's being fed to the motor in a steady supply.....After six years of running, and troubleshooting multiple "stumbling engine" issues either in my own car or alongside my fellow cometitors, fuel supply (or lack thereof) became the number one culprit in the engine stumbling/laying down scenario.......Not sure if you you've been tank-diving lately, but a dirty sock on the fuel pump, a deteriorated "S" line on the pump, or a clogged fuel filter are all possiblities. To that end, I run a fuel pressure gauge on the dash in my car to immediately confirm/refute any incident of engine stumbling...... Failed or weak dizzy has been known to create issues too......Good luck with it and keep us posted.

[avgjoe]

Tony,

Any resolution to this?

[FFRSpec72]

Tony,

Any resolution to this?

Nope, ran into it again last week in Spokane which cost me a few positions. So one thing that I was wondering is if this could be a TPS issue, since I'm coming down off high revs into a corner (I have not tried just going high RPMs straight and then braking hard and then accelerate to see if this happens w/o a corner involved) with no foot on gas and then hard on gas, the final option is to pull the rear body and pull fuel pump to see if anything is wrong there since.

[mhoward1]

Do you happen to know what RPM the stumble occurs?

[FFSpecRacer]

Tony,

I've had this occur a couple times and have found it is related to the amount of fuel I have in the cell. It occurs when going back to throttle in a higher G corner. For some strange reason, I now have to keep 2 extra gallons of fuel in the cell. I have purposely run low on fuel to verify this and it happens at the same fuel level every time. But you make it sound as if you have a ton of fuel.

I don't know if this has something to do with the larger injectors, more fuel demand, but that is the only thing that has changed. My other thought, I have just not had a chance to go tank diving yet, is it is something like Karen suggests with the intake fuel line, sock, or trap box.

[mhoward1]

Scott had a similar problem with Foam breakdown in the tank.

I also had a similar problem with a single wire alt that refused to make enough power for a full throttle run at 2500 RPM and below (IE, taking a slower turn in 3rd instead of 2nd because of the position it would leave me in at the shift point).

[FFRSpec72]

Do you happen to know what RPM the stumble occurs?

Coming off 5800 RPMs and into hard corners and down to 900-1000 RPMs and hard on throttle, engine will mis fire (cough, backfire, bog down) and then gain RPM and no issues until I hit same set of corners again. Car runs great otherwise, can get up to 138-140 mph at 5800 (or so). Car is fine on sweaping high speed turns.

[FFRSpec72]

Tony,

I've had this occur a couple times and have found it is related to the amount of fuel I have in the cell. It occurs when going back to throttle in a higher G corner. For some strange reason, I now have to keep 2 extra gallons of fuel in the cell. I have purposely run low on fuel to verify this and it happens at the same fuel level every time. But you make it sound as if you have a ton of fuel.

I don't know if this has something to do with the larger injectors, more fuel demand, but that is the only thing that has changed. My other thought, I have just not had a chance to go tank diving yet, is it is something like Karen suggests with the intake fuel line, sock, or trap box.

I have know that I have had at least 15 gal (gauge is not that accurate) in tank and had this to happen. I will pull fuel pump and see if I see anything odd there. I assume that the fuel pump (stock from Fuel Safe, new tank design) is enough to supply the engine?

[FFSpecRacer]

Fuel pump for our cells is more than adequate.

B

[FFRSpec72]

Well took the fuel apart, all looked normal, the sock was OK, so was filter and the foam was fine no isues, but the collection chamber in the cell is very very small and I can see that on a hard trund unless tank is half full you can empty chamber

temporary-3.jpg

[johngeorge]

Tony, you should have a sump box like this in there.. (minus all that foam clogging the pump and the check balls)
Verify that the filter at the end of the fuelpump is not clogged. and while you are in there, take that sump box out and test to make sure it actually keeps fluid in there (we had another style sump on #85 that had trap doors and fluid was rushing past around the doors where it was mounted to the sump box and would empty box fast)

[FFRSpec72]

I pulled the box out and there was no foam blockage and fuel sock (filter) was not blocked. I did not test trap doors (as I did not really know what I was looking for beyond foam/filter issues), I will have to pull it again to test that, but since this box is so small I can see that any loss can cause problems. Thanks for the pointers.

[IEMRacing]

I put my fuel pressure gauge where I can see it.
It’s the little black gauge next to the center bar.
It’s a good trouble shooting tool when a engine is cutting out.

JT
iem8802.jpg

[FFRSpec72]

I put my fuel pressure gauge where I can see it.
It’s the little black gauge next to the center bar.
It’s a good trouble shooting tool when a engine is cutting out.

JT
iem8802.jpg

Is that a mechanical or electronic guage?

[IEMRacing]

Is that a mechanical or electronic guage?

It’s mechanical and comes right off the
Adjustable Fuel Pressure Regulator.

Hope this helps

JT

[FFRSpec72]

I ran with 10 gal of gas this weekend and still had the stumble even on the first lap, not bad but still there, got worse as tank levels got lower and lower so there seems to be a fuel delivery issue. It will have to wait until end of season to see what the issue is with fuel cell as we have back to back races for next 3 weeks.

[avgjoe]

Check your hoses and fittings are tight to make sure you are not sucking in air somewhere. Check your vent is clear and sized appropriately. People have had problems with the fuel hose from the pump to the bulkhead fitting in the cell splitting and not sending enough fuel and/or sucking in air. Verify your TPS voltage is smooth all the way to WOT. Check your connection to your MAF is good from the MAF back to the EEC.

We're smarter than these cars but they tend to have more patience and have no sense of frustration.

Joe

[FFRSpec72]

Check your hoses and fittings are tight to make sure you are not sucking in air somewhere. Check your vent is clear and sized appropriately. People have had problems with the fuel hose from the pump to the bulkhead fitting in the cell splitting and not sending enough fuel and/or sucking in air. Verify your TPS voltage is smooth all the way to WOT. Check your connection to your MAF is good from the MAF back to the EEC.

We're smarter than these cars but they tend to have more patience and have no sense of frustration.

Joe

So I'm running the stock hard fuel lines for a MkII (like maybe 5/16 pressure and 1/8 return). All fittings are tight, no leaks, the TPS has been replaces and I have .98 volts to TPS and seems smooth all the way TO WOT but will check again. I have a new harness this year and the MAF seems to be fine, no error codes reported on MAF.

[FFRSpec72]

Well time to remove fuel cell, car was running OK this weekend (normal amount of stumble) and then 3/4 through the race car was hard to keep going, was passed by every car on the track (was embarrassed down 15 sec a lap), just had to finish race and not get a DNF. So now the car takes a long time to start, takes like 10-15 key on/off to get to 40 lbs of fuel pressure, so something has gone south in the cell and most likely the problem from the start. So will have to venture into the cell this week and see what is going on.

[FFRSpec72]

So I think I found my problem, not sure why I did not see this before when I went into cell, but here is the issue I found, not sure of the cause. Also not sure how get the fuel sock/filter off the pump (as I don't want to break anything. This is the tube/hose that goes from the pump tube to the underside of the top plate.

Any thoughts on the cause? I assue that I was loosing fuel back into the cell and not getting it pumped to engine. On hard corners it was just having a hard time keeping the fuel pressure up due to the issue in the picture.

[FFRSpec72]

I called Fuel Safe and they indicated that they recommend to replace the Fuel Pickup Line every year to 2 years, so I ordered new line and filter (as filter was really bad), along with a new gasket. I rinsed out bladder with simple green along with the foam, let all that dry, put in new section of fuel pickup line and put it all back together, and now I have fuel pressure again.

As a side effect seem that with the fuel pickup like the way it was I was leaking fuel back into tank, as I was dropping fuel pressure to zero about 45 seconds after I turn engine off, now I can maintain about 20lbs in the lines for at least 48 hours (as that has long its been since I got system back together).

So hoping this solves my stumbling issue now.

[mhoward1]

I hope this fixes your issue. You have been dealing with this for a while.

[Scott McKay]

A working fuel pressure gauge within safe eyesight of the driver is a mandatory device in trying to solve a problem. Trust me.

[FFRSpec72]

A working fuel pressure gauge within safe eyesight of the driver is a mandatory device in trying to solve a problem. Trust me.

Yep, a Speedhut fuel pressure gauge is going in this winter, I learned my lesson !

[skullandbones]

Glad that you exercised your fuel pressure demon!!! I've been following the thread faithfully to see what the final outcome would be. Having an EFI but not a spec racer, there is a bunch to learn about the system. I guess there will have to be a second gauge added to the pod besides the A/F ratio meter that Karen also recommended. One thing I've noticed is that you guys seem to lay out your dashes very carefully not to be distracted with a bunch of dials directly in your face unlike on many of the street roadsters. Good luck racing, WEK.

[FFRSpec72]

One thing I've noticed is that you guys seem to lay out your dashes very carefully not to be distracted with a bunch of dials directly in your face unlike on many of the street roadsters.

One thing about the SpeedHut gauges is they have a programmable warning light so its just a quick glance of the dash for any lights

[skullandbones]

I agree. The light should get your attention like a check engine or high beam light. One of my first upgrades is to redesign dash and change gauges (used autometer ones in kit). I will be looking for that feature when I do. Thank you, WEK.