vinny R

Re: Tpi Fuel Pressure
The factory fuel pressure with the engine running and the vacuum line removed & plugged from the fuel pressure regulator is 43psi. Forget what TPIS might say on the matter. Start with the factory fuel pressure and then tailor to your specific mods. 32psi is too low, allowing the engine to run lean, then the ECM is going to try to richen things up alot. You might run out of correction room allowed in the ECM.

Hi Guys I found this post from 8 yrs. ago. I wanted to know if this info is true. I am having a stalling issue with the car that I think is the Fuel Pump. I hooked up a pressure gauge and idling in park I am 33PSI. If I rev the engine to 2500 and hold the throttle it drops to 30. The post above mentions the vacuum line to the reg being disconnected. All the others posts I found seem to say 43 PSI with the car running and mention nothing about the reg. being disconnected. My "****" dyno also tells me I am down on power.

So I want to make sure I should have 43 PSI running with the reg hooked up. From what I found this sounds right but from searching past threads I found some conflicting info.

The last sentence of the original post is what I believe is the reason it stalls.
This problem did start the next day after I got some gas. I drove home the 3 miles from the station no issues and the next day it started. Maybe I got crap in the gas and it clogged the "sock" on the pump?

vinny R

I found these numbers on another forum...I assume these would hold true on a 91 305?


1986-to-1990 5.7L/350 MPFI specifications

With full vacuum applied to the regulator (idle):
Use 38psi which is the average of 30.5 and 44psi.

With no vacuum applied to the regulator (WOT):
Use 43psi which is the average of 40.5 and 47psi.

Abubaca

iTrader: (3)

Hey Vinny, what's up!?

The 43 psi number everyone talks about is for wide open throttle. You test/simulate this by removing the vacuum line. You're essentially simulating vacuum conditions inside the plenum, and therefore vacuum conditions pulling (or not) on the diaphragm inside the regulator. At idle, 33 psi sounds pretty good, with all vacuum lines connected. That's right where I'm at too. What you really need to do assuming you don't have an in car gauge is see the pressure under load. Hook up the gauge, tape it to the windshield and drive around.

Fwiw, while you CAN test WOT by disconnecting the vaccum line, you need the car driving and under load to test off throttle. Ask me how I know. Just like you I'd rev it in the driveway, and watch the PSi go from 33 to about 35, then fall back down, even dropping below 33 by a few PSI. Someone mentioned that it needed to be under load, and sure enough, while driving the RPM and PSI climbed together. In the driveway simply revving they did not.

EDIT ---- I feel pretty confident about the 43 psi at WOT and the test procedure...and also about testing pressure under load as opposed to driveway revving. That being said, I'm not sure really where the "LOW" psi at idle should be. My L98/700r4 idled at about 35. My current 383/TKO idles at 33. Both ran/run fine. That's not to say that for you, 33 isn't a tad low. Honestly I don't know. ....but I'd start by driving around and confirming that you ARE increasing PSI from 33 to 43 while accelerating under load. I imagine you are, and then you're probably NOT looking at a fuel pump issue. Still, dropping down to 33 psi might be an issue causing a stall....but I don't think it's fuel pump at that point. Anyone care to check my math/logic on that? That's how I'd attack the problem anyhow...

vinny R

Hey Chris...Livin the dream!...Good to hear from you.
I will try taping the gauge to the windshield a drive around the block, that is a great idea. I will get back with you on the results. The thing I see right now is when I engage the throttle The pressure spikes to 40 and then immediately falls to 30. If I keep the throttle engaged it holds steady @ 30. When I let off the throttle the pressure goes up to 33. I am really curious to see if it acts the same under load.


One other thing, do you know what the proper leak down numbers is on the pressure gauge? When I did the last test it took less than a couple minutes to leak down.

Abubaca

iTrader: (3)

ehhhh not sure about leakdown...but I know mine have always taken a good bit of time. WAY more than just a few minutes

armos

I think the reason for disconnecting the vacuum line must be to eliminate confusion caused by variations in manifold air pressure. The fuel pressure is supposed to stay constant with respect to air pressure in the manifold, since that's the pressure that the injectors are spraying into.
This means that when measured with respect to the outside atmosphere it will be seen as fluctuating. But those fluctuations should be in sync with what you'd see on a manifold vacuum gauge.

With the vacuum reference hose disconnected, the fuel pressure will instead be referenced to the outside atmosphere and should appear to stay constant on the gauge. This isn't ideal for the running of the engine but it might make it easier to see if the fuel system is hitting the limit of it's capacity under load.

With key on engine off, manifold pressure = atmospheric pressure and the gauge should show about 43psi.
A long time ago I checked this on a 2.8L, and with engine running at idle (and hose connected) I think it only dropped a few psi. Dropping into the low 30s sounds extreme to me, but maybe I'm just remembering wrong.


re: leakdown - again it was a long time ago, but I agree with abubaca. I remember my 2.8L lost something like 5PSI in about 15-20 minutes. It sounds like yours is leaking down faster than it should, but it might not be leaking fast enough to be a big problem yet.

Tuned Performance

iTrader: (92)

30 seems low but I’m not sure it will cause stalling at idle I think that’s one of the symptoms you said you had. Leakdown its normail to have around a 5psi drop in 10 min or I have seen complete drop in 5-10 min.

vinny R

Thanks Guys...I am gonna try to disconnect that regulator and see what happens on the gauge. I am also going to check with the gauge taped to the windshield and see where it is when driving(slowly). I will let you guys know what I find. I am the 8th owner of this car so it just could be the original pump for all I know.

Abubaca

iTrader: (3)

Just to be clear...

You test while idling in the driveway with the vacuum disconnected to SIMULATE Wide open throttle vacuum. ....but while driving, have the vacuum line connected as you're not simulating anything.....you're reading ACTUAL pressure.

...and what you're looking for is that pressure rising as the rpms rise, dropping as the rpms drop. Range being low/mid 30s to low/mid 40s. ....specifics beyond that are hard to tell without datalogging.

vinny R

O.k. I did some testing. With the reg disconnected and idling in park it is @ 43 psi.
with the reg. connected and me driving under load it was 33psi at idle and 40 or so when I hit the gas. It drops back to 33 when I let off the gas.

That all sounds normal right?...when driving with the reg connected I went around the block. I hit the gas to go, not punched it but a little more than normal acceleration and it took off for about 3 seconds backfired and died. Started right back up drove it home, maybe got 1/4 mile tops and it died at idle in the driveway.

Next I disconnected the reg and drove the same around the block. The gauge pretty much stayed @ 40-42 PSI. At one point it stumbled and seemed to want to die but did not. got home in the driveway and she died at idle. It also died at idle when I was running the engine to warm it up before testing. It idled fine until it started warming up.
Then it would die with or without the reg hooked up.

A couple things, first is back fire a condition of lean? The exhaust smell seems different since this issue. When I remove the gas cap there is no pressure release from the tank. I tried a different cap and no pressure. The fuel pump sounds quieter. When I stand at the back of the car I used to hear the distinct sound of the pump running. I just went and fired the car again and could barely hear it and that was with me under the side of the car, she idled for about 15 seconds and died. The reg was disconnected and the gauge reads 40 psi. the car is up to temp also. Another observation is the problem has gotten worse in 2 weeks. I was able to drive a few miles before it would happen and only a few times would it die. Last week it was more frequent and the backfire started. Today the back fire persists and now I can't even get around the block.

Let me know your guys thoughts on this.

vinny R

my buddy just stopped by with his 92z. We did not put the gauge on his as it was too hot.
But what I did do was open his gas cap and release the pressure. we started his car and let it run for 3 mins and released the cap and pressure came out. I put his cap on my car and it ran for about the same time, then died, released the cap and absolutely no pressure. Not sure if this has anything to do with my problem but it sounds like it is a problem as I should build pressure and I am not.

Abubaca

iTrader: (3)

The fuel pressure issue seems to be fine. One pound here or there isn't gonna make a difference, and those numbers seem fine.

.....as for the pressure in the tank, I can't directly point at one thing, but that could certainly be an issue. Maybe the pressure is only affecting the pump enough to make a difference at idle. Even a little hiccup may be enough to cause a stall condition? I'd be hitting up the search button!

PaulyC

Just a clarification, the fuel pressure will rise as the vacuum drops not necessarily as rpm rises. Under load we see a drop in vacuum and therefore need a rise in fuel pressure to supply the added demand. What you are seeing seems normal to me. I also wouldn't be too concerned with the air pressure in the tank as that is more of a function of the emission control system. I am feeling like you may have a different problem. Does the MIL come on when this happens or is it on when running? Check your TPS voltages and MAP sensor also to verify correct feedback. And last, do you have a scanner you can connect?

vinny R

What is the MIL? If it is a light on the dash than no. My dash is a junkyard cluster with 252K on it and the only light that comes on is the INFL when you turn the key on. The TPS is new within a little over year ago. I believe the MAP to be original. I will look into the test for it. The 2 things I notice is the smell of the exhaust is different when it dies and the fuel pump is quieter. I do not have a scanner either

Abubaca

iTrader: (3)

This is why you need to drive the car, not simply rev in the driveway.

Abubaca

iTrader: (3)

Can you MAKE the car stall? ....I'd be curious what the fuel pressure is doing while this happens. ...although I suspect nothing out of the ordinary.

I'd be curious where that tank pressure is going.

red rock

iTrader: (2)

You mentioned earlier about it not holding pressure for very long. You may have a leaking injector. You may want to ohm the injectors. Here's a link; https://www.thirdgen.org/forums/tpi/...injectors.html

vinny R

Yes the car stalls on its own. The pressure bounces a little as it is stalling. But it seems just to move with the rpm's as it stumbles and stalls I does not just drop on the gauge and then stalls.
I did drive it with the gauge hooked up and my results are a couple post up

vinny R

Yes I did ohm the injectors and all was 17.2 plus or minus .1 in either way. They are the original Multecs.


This problem did start the day after I got gas but I think it is a coincidence as the problem has gotten worse over the past 2 weeks. it stalls more frequently then it did when this started.

red rock

iTrader: (2)

If you suspect bad gas, try a can of Sea Foam or gas treatment of your choice. When was the last time you changed the fuel filter?

vinny R

I changed it about 3 yrs ago when I first bought the car.

red rock

iTrader: (2)

I guess with possible bad gas, I would change it and treat the tank. Rule out the obvious first. The injectors will thank you also if one or more are clogged.

vinny R

One other thing, I did put a aftermarket hypertek chip in that was used. It has been in a month or so with no problems. Maybe something has changed with the fuel mapping and the computer does not like it? I think I may take that out also.

red rock

iTrader: (2)

Yes, I would take it out and try it. Most of those aftermarket chips can be an issue. Especially if it was used.

Pro

fyi I posted the bin for that hypertech chip in other thread and all they did was adjust TCC and spark advance at like 4500rpm

Tuned Performance

iTrader: (92)

It’s a 121342 not sure if the file is corrupt or not. I did a read from a memcal I have.

Tuned Performance

iTrader: (92)

incase anyone wants to view the file. There are more areas than tuner pro that show changes to that are unknown to me compared to axxb.
@pro you posted a 122322 file
https://www.thirdgen.org/forums/tpi/...p-what-do.html

vinny R

I put the Memcal with the original chip back in place and still the same. So the chip is not it. It sure has a very distinct smell when it dies. I think lean, kinda chemically. My O2 sensor and TPS sensor are both fairly new and A/C Declo parts. I did a search on MAP sensor test and will try that but it said symptoms of a bad sensor is rough running idle and rich. Below is a pic of the label from the ECM. It is a reman and it looks like it got wet? Is this normal looking.

Abubaca

iTrader: (3)

I know we're jumping around a bit, but I still think it has something to do with the fact that you have no in tank pressure. Whether that's the problem itself, or simply a symptom of a yet undiscovered problem. I figure start with what you KNOW isn't right, regardless of whether or not it's the actual problem.

Tuned Performance

iTrader: (92)

I would look into why the check engine light doesn’t work. I have had a few ecms that are reman units and the sticker looks the same. Unless your heater core blew at one time getting the ecm wet.

Pro

Another reason to cap it off if you live in the south.

W.E.G.

I'm not much of an expert on this, but I'll throw my two cents in here.

Your fuel pressure can be "fine" (within pressure range), and you can still have fuel pump issues if the ELECTRICAL CONNECTION to the pump, or within the pump, is imperfect.

You can be driving along just fine and if the electrics to the pump (or to the ignition!) go intermittent on you, I believe you'll get the "bucking bronco" behavior.

Which will have you pulling your hair out trying to diagnose.

Ask me how I know.

Pro

how do u know

vinny R

I would guess you would smoke test for this?




I would look into why the check engine light doesn’t work. I have had a few ecms that are reman units and the sticker looks the same. Unless your heater core blew at one time getting the ecm wet.


No it just leaked. I would not know where to start to figure the light issue. I seen the back of my original cluster and that is beyond my knowledge of cars.

vinny R

I agree with what you are saying but in my case the car has died when I have the gauge hooked up and the pressure does not drop. If the pump was shutting off wouldn't it lose pressure as the engine dies? Or wouldn't I at least see the gauge fluctuate before it dies?
And yes how do you know?

PaulyC

We really need to check vital signs or all of this is just a guess and you will be throwing parts at a problem with hopes that one works. Your fuel pressure passes all of the tests, tank pressure is primarily relieved/controlled by the evap system (if you still have one) so unless it’s real high or under a vacuum I don’t see that being an issue. It may not be right but it won’t have this affect. That is supported by the fact that the pressure at the rail passes. Next, even if you say some of the parts are new they may need to be tested. New parts can fail just like old parts can seem to run forever. Last, maybe you can get a cheap code scanner if you light doesn’t work, a trouble code would really help out now.

W.E.G.

I know because a have one of these cars.

It has a lot of original parts still on it.

My troubles are intermittent.

Sometimes I get codes. Sometimes I don’t.

W.E.G.

Now, an inventory question for the OP, in case I missed it in the preceding discussion.

What original system parts have been replaced?

Focus on fuel and electrical.

Also, have any original system parts been altered or deleted?

Now is also a good time to inspect all vacuum connectors and connections. Same with electrical.

Challenging as it may be, it becomes necessary to inspect and test every circuit on these old cars until the culprit is found.

And don’t be surprised if the culprit eludes you on first examination.

That is how I know.

vinny R

I am going to look into a scanner. I think that would help yes. And I will test the TPS.


From what I can tell all the original fuel system is there including the whole canister. There is a valve at the tank and one before the canister. Original injectors and EGR system. Original idle air system. Can't tell whether the pump was replaced or not. For Electrical it is a reman computer. The only system that appears to be "hacked" is the fans, The ground wire from/to the relay has been cut and a jumper wire to a ground, so the fans come on with the key. At least I think this to be true. This has been this way since I bought the car 3 yrs ago.

W.E.G.

The Moates ALDU1 and TunerPro RT will give you some more info.

But will not be lots of help with intermittent issues.

Especially if the issue is not a system monitored by a sensor.
For instance, the “scanner” won’t tell you when you have temperature-induced restriction in fuel delivery, or if your coil wire is intermittently arcing to ground.

The OBD-1 scanner is only going to give you numeric values on sensors, and report stored codes. It won’t tell you if you have an injector gone bad or misfire in a particular cylinder.
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W.E.G.

You mentioned backfiring.

Backfiring is often related to timing.

Have you had a timing light on the motor?

If so, does the timing mark stay in one place, or does it run around like a rat on crack?

vinny R

No I have not had a light on it since I bought it about 3 yrs ago, I will see if I can find my timing light and check that also. I am hoping to have some time later today.

Is the timing set in closed loop? I know the computer advances the timing when in open loop. Should I test when hot?

W.E.G.

The key is to disconnect the Electronic Spark Control wire when running the light.

It’s a brownish wire, usually protruding from the wiring harness bundle, on passenger side, at top of firewall. Should have a click-clip connector.

That ensures that all systems are operating except spark-advance.

Your base timing on a V8 should be 8 degrees BTDC.

If the timing mark still jumps all over the place, you have slop in your timing chain and/or the elastomer bond has failed between the hub and the weight on your harmonic balancer. Also possible, but far less likely, you have slop between your distributor gear and camshaft gear.

Timing too advanced will knock. Timing to retarded will backfire.

Blowing too much unburned fuel into catalytic converter will degrade and clog the converter.

W.E.G.

You can set the timing in whatever loop status you want. The only difference between open loop and closed loop is that the ECU reads the O2 sensor, and reacts to its input across the fuel map, and alters injector pulse, in closed loop.

Because base timing is checked at idle, the ECU won’t be doing anything dynamic with injector pulse.

I would check timing with the motor hot, since you are probably going to end up having to time it more by ear, than by that jumping-all-over-the-place timing mark.

Try to find a sweet spot where it sounds smoothest (or least rough). The timing light will get you in the ballpark on a tired motor. Your ear will get you to where you need to be.

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Pro

I wonder if the timing chain is stretched and timing is jumping around all over the place. Theres another video on youtube somewhere with a chevy 5.7 truck and it was doing what you say it does - runs sometimes, randomly dies, but fuel pressure is good. Runs just well enough to drive it around the shop and pull it inside. And it turns out the chain was stretched. They replaced it and all is good.

79Daytona

You know my 92 Z28 5.7 TPI is doing that bucking bronco thing when giving it gas from time to time and I just put a new gas tank and sending unit and fuel pump and a fuel pressure regulator. Oyeah a new fuel pump relay. I'm starting to wonder if I got a bad sending unit because the fuel gauge doesn't work now.

W.E.G.

It isn’t going to run well if the timing chain is stretched, and your valve timing is constantly changing. All the worse for setting base timing if the chain has jumped a tooth. The jumped tooth thing is why you hear me always saying you should fine-tune your base timing by EAR.

If you can keep constant tension on a stretched chain by maintaining consistent throttle and consistent load on the motor, the herky-jerky may “settle down” a bit. Then when you are on-and-off the throttle, the ECM ignition timing advance gets out of sync with the timing-chain-slop. The slop causes wavering valve timing, and the bucking bronco effect can reappear.

Remember, there are TWO types of timing working concurrently, and they have to be in sync with one another. Electronic spark timing. And mechanical valve timing.

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nomind

I had a similar problem on a Focus after I swapped out the pump. My problem was a loose plug. Maybe try and separate the plug infront of the tank, and spray some electric contact cleaner. Or it could be the connections at the pump if you used any of the old wiring. I had that problem with a 92 V6. Spent the better part of a day going through that wiring, and the problem was the in tank connections.

79Daytona

That a good point I never thought of that. The car from what I can tell has set up for some time. Thanks again for the info.

vinny R

First of all thanks to all of you guys for helping me out with this. The problem is solved. And I believe it was W.E.G. that said it could be a part that I have replaced already. Well it was a part I replaced 3 yrs back when I first bought the car. It was the ICM. And yes I overlooked that because I said to myself that I replaced it and surely it would not go bad in that short amount of time. The rotor was burned at the top were the connection to the cap is so I replaced that also. It had a ton of corrosion down in the bottom of the dizzy, Living in SoFlo the humidly just breeds corrosion. The car runs great, better than before as this must have been a slow failure of this over time. I reinstalled the chip I bought as I did like the way the car performed with it. Now I will tackle the leak in the EVAP system.