I just replaced the fuel pressure regulator on my 93. When I turn the key to start it my fuel pressure goes to 41psi but when it fires up it drops to 34psi. It stays @ 34 until I shut the car down then it goes back up quickly & holds @ 41. Is that normal if so why does it do that?
I replaced it because before it would only go up to 34 then drop to 19 like a rock.

 

I believe this is normal when the vacuum line is hooked up to the regulator. If you check the pressure while disconnecting the line it should go back up to 41.

Doug

 

I'm pretty sure that's normal pressure for a LT-1. Does it stay at that pressure for at least half an hour and then slowly drop after turning off the engine?

 

It did. Thanks. Is there any advantage to removing the FPR vac line capping it & capping the FPR inlet. Ive read that a lot of guys run that way. Will it damage anything?

Thanks

 

Yes it does

 

I'm wondering where you guys are getting these time periods that fuel pressure should hold without dropping. Now I see it's half an hour; where's that coming from, the source?

Thanks,

Jake

 

I don't think it will damage anything, all it does is supply more pressure to the injectors so my guess is that the injectors will have a shorter duty cycle since they will be spraying more gas because of the increased pressure

 

JAKE, I read that out of my repair manual. some say an hour, and some say half an hour.

 

Since the issue of holding pressure comes up so often on this and all the other Forums I visit too, I've wanted to know if there's a time period associated with it.

In all I've read I've never come across any and that includes in any of the 100 lbs+ of auto magazines, manuals I have and in all the reading I do on different on-line sites.

I've even mentioned in previous posts of mine, that I've never come across any source stating a time period for the pressure holding. That sort of surprised me too, since I would think that someone, somewhere would have addressed that.

If you can recall the manual you read it in I'd really like to know. That way I can pass it along to others as the source.

I recall many years ago on the 86 I had at the time, I sat and timed the pressure drop - you know, how many psi in how many minutes. I posted the chart of my results here, but pressure certainly didn't hold for even a half hour. Wonder if that chart is still archived here?

IIRC, pressure would hold steady for a few minutes then the pressure would begin to drop in like 2-3 psi increments over a 3-4 minute time period but it wasn't linear. Pressure dropped more at some points than at others. Seems it did take around 1/2 hour (or so) to drop back all the way to zero, but mine certainly didn't hold steady for a half hour THEN begin to drop. I'm going from memory on this on something I did close to ten years ago so I can't be more precise.

Thanks,

Jake

 

Leave the fuel pressure regulator's vaccuum hose connected, your car will perform better at part-throttle.

 

Here are some things I came up with recently:

In diagnosing a problem my son was having with his 96 LT1, we (actually he did all the work, I just told him what to do) ran several tests. One was fuel pressure.

With the FPR vacuum line disconnected and plugged his pressure was 43psi, which is within the 41-47 psi spec. With the vacuum line connected, pressure was 36/37 psi. The hash marks on his vacuum gage weren't large enough to clearly distinguish the exact number.

As a Rule of Thumb, pressure changes 1 psi for each 2 inch/Hg of vacuum.

Now, tests done by TPIS and reported on in their Insider Hints booklet show that LT1s like 45-47 psi with the engine in the car. So even with the vacuum hose to the regulator disconnected and plugged Ryan's engine couldn't get there, psi-wise, LOL Hence the need for an adjustable fuel pressure regulator. The above high-lighted paragraph is wrong. What TPIS did doesn't apply to the LT1 engines. Those tests were done in 1987 before the LT1/4 was introduced. My mistake for not noting that before I posted and my thanks to STL94LT1 for catching my error and pointing it out to me. I left it in this post, rather than deleting it, so that any L98 guys can see what TPIS says about THEIR engines.

More on point: I don't see any downside to running the engine with the vacuum to the regulator disconnected and plugged. In fact that's what I told Ryan to do so his engine would be running with 43 psi continually, which is still within factory spec. The injector spray pattern should be better, too with the higher pressure.

I recall that when I played the "increase the fuel pressure" game on the 415 I had in my 86, the engine felt smoother with the higher pressures. If radiated header and under hood temps are any gage, the higher fuel pressure made the engine's headers run cooler and the under hood temp was noticeably lower. Now's the time I wish I had used a temp gun so I could post the actual before and after numbers.

Hope this helps.

Jake

 

Jake, not trying to start an argument here. But, in the TPIS 2003 catalog they claim that unless reprogrammed the LT1 will be faster at a lower fuel pressure (39) psi.

Also, the stock PCM is calibrated to run best at part-throttle with a lower fuel pressure and the vaccuum hose connected. Once you start reprogramming this can change.

 

You know what, I believe you're absolutely right about that. Here's what I was referencing and how I made the mistake:

After reading your post I went back to check my facts. Here's the quote from the booklet:

"Back in 1987, when we ran extensive fuel pressure tests (see Chart R) we found that the stock motor with dyno headers installed seemed to like 42 PSI on the dyno and 45-47 PSI when installed in the car."

I had high-lighted that section in yellow because, back then, I had a 86.

What I missed and by going from memory was the "Back in 1987 . . ." part, which was long before the LT engines were even introduced. So what followed in that section of the booklet doesn't apply to the LT engines.

Thank You Very Much for catching that. I don't have a problem with being corrected, in fact, I really appreciate it. Keeps more "bad" information from being circulated.

I'm going back now to edit what I posted.

That outta teach me not to rely on memory for things like this.

Thanks again.

Jake

 

Here's how I look at fuel pressure:

There are generally two schools of thought; those who prefer larger injectors but lower fuel pressure and those who like the opposite - smaller injectors but run at a higher pressure. This is kind of an on-going thing, with each camp citing reasons their approach is best. What else is new.

I don't have TPIS's article on the test results done on the LT1, but I'd sure like to read it (hint, hint). Since their tests results, apparently, run contrary to what I'm about to write (below) I'd like to have a look-see. The part throttle spray pattern at 39 psi has got to be worse than the pattern at a higher pressure.

Anyway, I'm in the camp of the higher pressure/smaller injector. I see it this way:

The PCM is programmed to attempt to maintain (in Closed Loop) a 14.7:1 AFR, and, within it's ability to do so, will always attempt to achieve maintain that. Obviously if the fuel pressure was set to, say 500 psi, the PCM wouldn't be able to compensate, but within a realistic pressure range the PCM can and will compensate.

When running higher fuel pressure, more fuel is injected for a given amount of time than is injected at a lower pressure. As we up the fuel fuel pressure, the PCM will receive feed-back which it will interpret and as a result will shorten the injector "On" time to compensate in an effort to achieve and maintain 14.7:1

So we end up with higher fuel pressure, lower injector "On" time but the same 14.7:1 AFR.

The benefits, as I see it, are a better injector spray patten at the higher pressure and the lowering of injector "On" time. The improved spray pattern may also improve gas mileage, but that's right foot dependent.

Of course those in the other camp will disagree and Myron's booklet makes mention of a competitor who's not in the "higher pressure" camp and who's against the higher pressure approach.

Just my thoughts.

Jake

 

I'm in the same camp!!

 

The 41psig fuel rail pressure with the engine not running is in the normal operating range. As for the pressure drop when the engine is running, this is also normal. The electric fuel pump can deliver more than 60psig dead headed (no flow), so the fuel pressure regulator is needed to reduce the fuel pressure to the design operating pressure of the fuel injectors.

As stated on other posts, the fuel flow rate of the fuel injector depends on the pressure (D/P) across the injectors. More pressure gives more flow (within limits) and less pressure = less flow.

To maintain a predictable fuel injector flow rate under all engine operating conditions and loads, the fuel pressure regulator is designed to have the fuel rail pressure track the manifold pressure (actually vacuum). With the engine not running, or at full throttle, the manifold vacuum would be at minimum and fuel pressure at regulated maximum. At engine idle, the fuel rail pressure would be at the lower regulated pressure, somewhere around 8psi less than with the engine not running. This should explain the fuel pressure increase after you stop the engine.

The fuel rail pressure drop after shutdown, or leak-down rate, is another matter. In a perfect world, there would be no leak-down and resultant pressure drop. Unfortunately in the real world nothing is perfect. Consequently we can expect some (small) leakage at the fuel pump check valve, the fuel pressure regulator and at the 8 injectors. The actual amount of leakage depends a lot on the condition of the individual fuel system components. For this reason a leak-down time greater than 30 minutes is generally considered OK. A lot would depend on where the leakage is. If everything leaks through the injectors, or a leaky regulator diaphragm, you would probably have some amount of hard starting when hot. On the other hand, a leaky fuel pump check valve would be quite benign.

On the idea of disconnecting the manifold vacuum line to the fuel pressure regulator, I would not do it. With the ECU running in open loop mode, as during warm-up and heavy throttle (somewhere >70% throttle position), there is no closed loop O2 compensation and engine fuelling is largely uncompensated. Running like that on a stock engine, I would expect to run quite rich with no benefit other than burning more fuel. However, if you are running a modified engine and are limited on injector size and running lean, pulling the regulator vacuum line will help you to get a little more fuel. But that’s just a band-aid and not a real solution.

Just my 2 bits worth…