The fuel pressure gauge strikes again

#1

Last Summer I put in a dash mounted FP gauge to help me troubleshoot engine management issues. The idea was that if my engine started hiccuping, a glance at the FP gauge would significantly narrow the list of possible causes.

Since then the FP gauge has been a big help. There’s been lots of hiccuping in the months since and the FP gauge did indeed narrow down the list of possible causes.

Tonight it helped again.

Over the past 2 nights I replaced my engine harness. It’s a little more complicated than it would otherwise be because my injectors are individually connected to the harness and I had to abrade and apply conductive grease to every connection on the harness just like the one it replaced.

The harness had it’s own relays.

Tonight I attempted to fire up the car and it wouldn’t catch. FP guage said no fuel. First thought was that I’d reversed the CPS and spark sensor connectors. If no CPS signal to DME than fuel pump doesn’t get power. But reversing those connectors didn’t help.

Next I pulled the fuel pump relay and jumpered the pins. I could hear the fuel pump running and the FP gauge showed pressure. So I swapped fuel pump relays, the FP gauge showed pressure and the car fired right up. Then I put the suspect relay back in place to make sure it wasn’t screwing with me and no fuel pressure. Good relay back in and engine fired up again. The whole process wasn’t 5min.

A dash mounted FP gauge is your friend.

#2

I know it’s difficult to tell in the garage, but do you believe the harness fixed your problem?

#3

You are correct sir.

I’ve don’t know. The symptom is pretty darn subtle. I can see the weakness of the engine better by watching the tach’s climb rate slow dramatically once it gets up to a certain point, but it’s darn hard to hear. It’s hard enough to detect with my ears that I’m not going to bother revving it up in the garage.

I’m doing a test day at RR the day before the NASA event. I’ll find out then.

#4

A fuel pressure gauge can be misleading since the fuel pump relay is controlled by the DME. Anything that results in a loss of timing data to the DME, a brownout of the DME, or a fault in the DME can result in the fuel pump relay opening with a resultant loss of fuel pressure. Been there, done that…

#5

Oh for chrissakes, all a FP gauge does is give you FP. How can that be misleading? Sure, there’s several possible causes for FP problems, but the gauge itself doesn’t attempt to provide any insight into possible causes so it can’t mislead. It’s up to the pissed off and frustrated SpecE30 owner to figure out WHY they have a FP problem, if any.

Nothing is going to be perfect. What I attempted to do here is to illustrate how handy having FP gauge on the dash was, not to suggest that it was a general troubleshooting wondertool.

#6

Oh for chrissakes, all a FP gauge does is give you FP. How can that be misleading? Sure, there’s several possible causes for FP problems, but the gauge itself doesn’t attempt to provide any insight into possible causes so it can’t mislead. It’s up to the pissed off and frustrated SpecE30 owner to figure out WHY they have a FP problem, if any.

Nothing is going to be perfect. What I attempted to do here is to illustrate how handy having FP gauge on the dash was, not to suggest that it was a general troubleshooting wondertool.[/quote]
I completely agree that it is a good idea to be able to know what the fuel pressure is. I was just trying to caution against jumping to an erroneous conclusion from a seeing drop in fuel pressure. Or not seeing a drop…

#7

Ranger, did your hicuping problem feel like fuel starvation? And when did it happen? I had a similar problem yesterday at LMP. I thought it was a fuel pickup problem because it was happening after turns (like if the fuel sloshed around), but it happened on the main straight too. What were your hicuping symtoms?

#8

I had to write this up on bf.c last week, so I’ll repost here:

This is my experience of the engine stutter caused by a dying fuel pump: At first, it will only be with heavy throttle/high rpm after being run for a long time. The “warm up” time will continue to get shorter, and the throttle and rpm thresholds will keep getting lower. I’ve always replaced them before they completely die.

Hard cornering does make it worse, but it will be on the straights as well. Fueling is definitely a common issue with these cars. Many threads to be found here. Good luck, and I’m in CT if you need help.

#9

No.

My car will hiccup when it gets a little low on fuel. It’s different for each race track and only an issue for track days or should I do an enduro again.

The non-fuel related hiccuping I’ve experienced since then has exhibited itself in a number of different ways and subtle things like this are hard to describe. Fuel related hiccuping, to me anyways, feels like the engine cuts out for an extended period of time, say maybe a quarter of a second. To put that in perspective 5krpm is 83 rotations of the crank.

The hiccuping that seemed to be related to voltage problems was more subtle. At high rpm and WOT the engine would clearly miss and stutter until I backed off of the throttle a little. Imagine the engine not firing for just a couple crank revolutions at high rpm. You’d feel it and hear it, but there wouldn’t be the dramatic shifts of weight that are exhibited in the fuel hiccuping above.

This got a lot better when I tightened the alternator. Then I went thru every critical power and ground connection in the car, abraded the tarnish off of them and then put on some conductive grease. That’s the effort I was working on when I almost lost my darn finger.

The latest flavor of hiccuping is pretty subtle. Primarily it just behaves like a loss of power at maybe 4800 to 5800rpm. The engine, oddly enough, seems to come back to life just before RL. The easiest way to detect it is that the rpm’s rate of climb slows. Well, that, and the car in front of you starts leaving your ass. I’m not dead sure if there is a change of engine note when this occurs. Sometimes it’s hard to know for sure if you’re really hearing/feeling a difference or if you’re just imagining it.

One thing that helped was replacing the master relay. But I don’t know if it was because the relay I got from Levie was in better shape or if the fix was simply the reseating of the relay due to the swap.

#10

When chasing down these sorts of problems I’ve found that having a log of switched ignition voltage, DME power voltage, and tach signal is helpful. Switched ignition voltage drives the injectors and coil and is furnished through the main battery cable. DME power comes through the small cable and main relay. So being able to see those will pretty much tell if there is an power problem and a good idea of where to look. Logging the tach signal, which is generated by the DME from CPS data is also helpful.

It would be even better to have logs of all of the inputs and outputs for the DME. In most cases, a few minutes of examination of those logs will tell you where the problem is.