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Fuel Pressure/no run Your diagnosis? by Gwain
Started on: 04-13-2008 05:22 PM
Replies: 17
Last post by: tjm4fun on 04-15-2008 07:46 PM
Gwain
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Report this Post04-13-2008 05:22 PM Click Here to See the Profile for GwainClick Here to visit Gwain's HomePageClick Here to Email GwainSend a Private Message to GwainDirect Link to This Post
I have an 85 GT 2.8L. I've had the car about a year, put about 20k on her, and one day barely limped home and couldn't restart.

I've done quite a bit of clean-up, tune-up to her, but after all else, the same symptoms survive.

I can crank her over, and she fires a few cylinders before she just dies. I'm getting tach pulses, and module, cap, wires, plugs - all new.

I installed a fuel pressure gage, and fuel pressure dies off instantly!!

If I key on the fuel pump, it runs a couple of seconds like it should. If I crank it over, the oil pressure sender energizes the pump like it should when the pressure builds. I can also jumper the pump to run through the ALDL connector.

Under any routine pump energizing, the pressure comes up to about 10 lbs. then falls off. If I run the pump through the jumper, it pumps gas, bogs under pressure (if I put my finger over the line), and maintains about 40 psi on the pressure gage as long as it runs. Pull the jumper, pressure drops to 0.

I tried pinching the return line, ran the pump ... same symptoms. Pressure falls off as soon as the pump stops. That suggests to me the pump is faulty?

One last item. The car cranks, several cylinders fire then it dies. If I jumper the pump, it builds 40 psi, and if I try to start the car, same symptoms, fires then dies. The 40 psi pressure holds though through the cranking cycle, as long as I leave the pump jumpered, but yet the car doesn't run?

I should also mention, I had all the injectors, including the cold start injector cleaned and tested by Cruzin Performance, so I don't suspect leakage there.

My question is, if the pump is truly bad, even though continuous cycling seems to hold 40 psi, will the engine still starve for fuel and not continue to run?

What's your diagnosis?

------------------
Marc in sunny Titusville, FL


  • 84 SE son's car, loaded
  • 85 Coup w/V6 transplant
  • 85 GT newly on the road
  • 86 SE/GT "The Chameleon" - big plans!
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Firefighter
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Report this Post04-13-2008 05:29 PM Click Here to See the Profile for FirefighterClick Here to Email FirefighterSend a Private Message to FirefighterDirect Link to This Post
Clogged fuel filter????????? Ed
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James Bond 007
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Report this Post04-13-2008 07:02 PM Click Here to See the Profile for James Bond 007Send a Private Message to James Bond 007Direct Link to This Post
What do you mean....20K on her?Diid you allso replace the fuel pump?If not,Im guessing it's the fuel pump.

[This message has been edited by James Bond 007 (edited 04-13-2008).]

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tjm4fun
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Report this Post04-13-2008 07:40 PM Click Here to See the Profile for tjm4funClick Here to Email tjm4funSend a Private Message to tjm4funDirect Link to This Post
you have multiple problems.
1. your fuel pump relay or wiring is bad.
2. from taking the load of running the fuel pump, your oil pressure sending unit is now also bad.

now for detail, this is how it should work: you turn key on, ecm picks relay, fuel pump runs for 2 secs. you crank car to start it, ecm sees pulses from distributor and re-energizes the fuel pump relay. car runs.
The oil pressure switch is a safety to keep car running in case the relay fails. it should NEVER be needed.
if you energize the aldl pin and fuel pressure remains fine and car runs fine, you have some issue in the wiring, it has nothing to do with the pump, filter, regulator, etc.
After re-reading your post, pressure is good with the override, so you have a wiring issue. now if you get the prime shot, which you say you do, that 2 sec shot should give you 40 psi. if you lose the fuelpump with jumper off, and the prime did work, the ecm is not seeing the ref pulse from the dist, and thinks the motor is not running. I'm actually surprised it runs if this is the case, as it also referneces that pulse to fire the injectors! you may want to swap the fuel pump and a/c relay, should be near the air cleaner on the firewall. it;s possible too that the relay does nto hold, only works a few seconds and then drops. the pin on the aldl is tied to the output side of the relay, so it proves out the wiring to the tank/pump.
here is the diagram for that wiring:
http://img87.imageshack.us/...776/86fprelaypu1.jpg
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Dodgerunner
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Report this Post04-13-2008 09:29 PM Click Here to See the Profile for DodgerunnerClick Here to visit Dodgerunner's HomePageClick Here to Email DodgerunnerSend a Private Message to DodgerunnerDirect Link to This Post
There can also be a short rubber hose in the tank coming off the pump that can split and dump the fuel back into the tank as well as/or a bad pulse modulator. The 10psi reading might be an indication of that and the pressure dropping off quickly. There is also a check valve on the pump but I have never seen one of those fail.

[This message has been edited by Dodgerunner (edited 04-13-2008).]

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Gwain
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Report this Post04-14-2008 07:51 AM Click Here to See the Profile for GwainClick Here to visit Gwain's HomePageClick Here to Email GwainSend a Private Message to GwainDirect Link to This Post
Thanks for the responses. Some clarification and still one question:

 
quote
Posted by Firefighter 04-13-2008
Clogged fuel filter????????? Ed


I installed a new fuel filter. I've pumped gas through it.

 
quote
Posted by James Bond 007 04-13-2008
What do you mean....20K on her?Diid you allso replace the fuel pump?If not,Im guessing it's the fuel pump.


I've put 20k on the car since I bought it. Total car mileage is 134k (Sorry, didn't mean to get you excited). I too believe the fuel pump is suspect, but I'm confused by some of the symtoms.

tjm4fun

You raised quite a few items, let me try to summarize:

First off, I can't get the car to run under "any" circumstances! It will crank, it will fire off a few cylinders, and then it will die.

The fuel pump is energizing properly, that is key on, relay on, fuel pump runs for 2 secs. I believe that part of the sequence is fine. The problem is after prime, I see at most 10 psi fuel pressure, or 0!

The only way I can get any significant pressure is jumping the pump at the ALDL. In this manner I get 40 psi and that pressure holds through cranking, as long as I leave the jumper active. Disconnect the jumper and the pressure drops immediately to 0! It's the same whether I pinch the return line, or leave it open?

One question I had which was not answered, if fuel pressure is the problem, shouldn't the car at least start and run with the fuel pump jumpered and the pressure at 40 psi? I can't get the car to run this way?

I may very well have an ignition wiring issue, but I haven't for the life of me figured out what. I've replaced everything except the exciter coil and magnet in the distributor. The car does fire off initially when cranked, but then dies immediately. After my clean and rebuild, so far the only thing I can measure and identify as a problem is the fuel pressure, 0 to 10 psi. unless I jumper the pump and let it run continuously.

Dodgerunner:

I'm familiar with the rubber hose problem. Fixed that in my son's 84 when we suspected a fuel pump problem. What confuses me is I can pump gas through the input line if I disconnect it from the intake manifold and jumper the pump, and if I hold my finger over the line while the pump is jumpered, I can feel the pressure and build pressure and hear the pump "bog" down under the load, yet I only get 40 psi max at the shrader valve with the pump jumpered, and with or without the return line pinched.

I believe there's a pump problem, but I'm confused by some of my diagnositc results and I'd like to be certain (if possible) before I drop the tank.

I'm also confused why the car won't start and run, even with 40 psi of fuel, though certainly there's a fuel problem from the pressure readings I'm getting.

Any additional thoughts? +'s for everyone by the way.

------------------
Marc in sunny Titusville, FL


  • 84 SE son's car, loaded
  • 85 Coup w/V6 transplant
  • 85 GT newly on the road
  • 86 SE/GT "The Chameleon" - big plans!
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Hudini
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Report this Post04-14-2008 11:11 AM Click Here to See the Profile for HudiniClick Here to Email HudiniSend a Private Message to HudiniDirect Link to This Post
So if you jumper the fuel pump the pressure goes to 40psi but as soon as you pull the jumper it goes to 0? When mine did that it was the fuel pressure regulator. The diaphragm had split. You need the special little star driver to remove the screws holding the FPR cover to the fuel rail.

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Report this Post04-14-2008 11:46 AM Click Here to See the Profile for JazzManClick Here to Email JazzManSend a Private Message to JazzManDirect Link to This Post
Take a test light and probe the fuel pump test terminal, connect the clip on the test light to a known good ground. When you turn the key to RUN but not crank, the light should come on for two seconds and then go off, that's the prime cycle from the ECM. When cranking you should see the light on continuously until you stop cranking, at which point it should turn off. If you are not seeing the light on while cranking it means your ECM isn't seeing EST pulses from the distributor, either because the module is faulty or there is a problem with either the wiring or the connector to the module. The tack sees the signal from the coil, IIRC the EST wire is separate and runs straight from the module to the ECM.

The fact that pressure drops off so fast also says to me that you have a problem with the regulator or the pump; the pump has a check valve meant to hold pressure, and the regulator stops releasing pressure at above 40PSI (with engine off). If you pinch the return line shut and cycle the pump to build pressure and then the pressure bleeds off rapidly then the pump is the likely problem, but to verify you need to pull the vacuum hose off the regulator and check for fuel. It should be dry and not smell strongly of fuel. The fact that the ECM primes the pump says that the ECM is working fine, the relay is good, and the associated circuitry is good to power the pump.

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Report this Post04-14-2008 01:19 PM Click Here to See the Profile for StockGTClick Here to Email StockGTSend a Private Message to StockGTDirect Link to This Post
One other test - Starting fluid could be used to verify that the ignition system is working. (keep fire suppression equipment nearby)
The engine should fire and run a bit, if the fuel pressure is the only issue.
If it does not start, could the pick-up coil in the distributor be failing, sending an intemittent signal to the ECM ?

The difference in fuel pressures when using the jumper to energize the pump is curious.
It could be the case that the failing fuel pump is slow to build pressure, and then loses pressure rapidly.

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Report this Post04-14-2008 03:27 PM Click Here to See the Profile for JazzManClick Here to Email JazzManSend a Private Message to JazzManDirect Link to This Post
The module sends EST pulses to the ECM, the ECM uses those pulses to trigger the coil via the module, with timing advance added as required. At low RPMs typical of cranking the ECM releases coil control directly to the module so it's possible that you'll get spark while cranking but not otherwise. The injectors will not fire at all unless the ECM sees EST pulses. For clarification, the ECM does not control the cold start injector, that is turned on and off by a mechanical thermal switch at the other end of the intake manifold.

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tjm4fun
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Report this Post04-15-2008 12:11 AM Click Here to See the Profile for tjm4funClick Here to Email tjm4funSend a Private Message to tjm4funDirect Link to This Post
I should have been clearer. Jazzman covered it.
you might have 2 problems, electrical and fuel. the fact that it will not run says ecm is not seeing the pulses. if you can get 40 psi with the jumper in, and the car is not running, then you need to address that first. then yuo can hunt down the rapid bleedoff problem.

From the 86 fsm, here is the relay circuit, I think the wiring is the same for an 85:


here is the top level of the dist to ecm for the ref pulses:



the top line to the ecm the ppl/wht is the ref signal line
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Report this Post04-15-2008 12:48 AM Click Here to See the Profile for merlot566jkaClick Here to Email merlot566jkaSend a Private Message to merlot566jkaDirect Link to This Post
fuel pressure regulator. check the vacuum line going to it, is it full of fuel? does it smell like fuel?


check the injector driver fuses too. you may have an injector(or batch) thats being comanded to be on all the time.
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Gwain
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Report this Post04-15-2008 08:35 AM Click Here to See the Profile for GwainClick Here to visit Gwain's HomePageClick Here to Email GwainSend a Private Message to GwainDirect Link to This Post
First off, thanks folks for some great inputs. This thing has been taxing me for some time now.

By the numbers: last night I put a test light to the fuel pump contact in the ALCL (Jazzman's suggestion). With key "on" I got a steady light for about 2 seconds. You could hear the fuel pump running at the same time. Turning the key to "start", with the engine cranking I also got a steady light (and the fuel pump clearly running at the same time).

If I understand correctly, and interpret the schematics correctly (thanks tjm4fun - I did invest in the Factory repair manual with my first Fiero - it's my "bible") that would seem to indicate the ECM is seeing pulses from the distributor, and since the tach is registering, from the coil too.

I'm going to "assume" this part of the circuit is good for now.

I tried giving her a shot of starting fluid (duh, thanks StockGT - wish I had been smart and thought of that! LOL) but she didn't act any different. Tonight I'm going to pull the air intake tube completely off, and try the starting fluid again. I'm also going to "sniff" the vacuum line to the fuel regulator, though I'll have to do that at the far end. I can't get to the regulator without removing the upper Plenum, and I really don't want to do that unless I have to since I just got the whole @#$$% thing back together!!

I'm going to put my hand vacuum pump to the fuel regulator vacuum line as well to see if it holds a vacuum.

merlot566jka: I'm going to check the fuses for the injectors. I've been wondering about them. I had them all cleaned, tested and rebuilt, but was uncertain how to check their operation on the car? If the fuse is blown, does that keep them "open" or "closed"?

Stay tuned for the results. Again, +'s for everyone.

------------------
Marc in sunny Titusville, FL


  • 84 SE son's car, loaded
  • 85 Coup w/V6 transplant
  • 85 GT newly on the road
  • 86 SE/GT "The Chameleon" - big plans!
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Hudini
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Report this Post04-15-2008 09:13 AM Click Here to See the Profile for HudiniClick Here to Email HudiniSend a Private Message to HudiniDirect Link to This Post
On the fuse question: The injectors are closed until the ECM grounds them. The power for the injectors comes directly from the fuse block. 12v is applied at all times the key is in RUN/START. The ECM only grounds the injectors in order to complete the circuit.

When you first turn the key to RUN, does the CHECK ENGINE light come on? This is supposed to verify power to the ECM.

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Gwain
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Report this Post04-15-2008 11:07 AM Click Here to See the Profile for GwainClick Here to visit Gwain's HomePageClick Here to Email GwainSend a Private Message to GwainDirect Link to This Post
Yep, the ECM check light is working as it should. Indicates the ECM is working.

------------------
Marc in sunny Titusville, FL


  • 84 SE son's car, loaded
  • 85 Coup w/V6 transplant
  • 85 GT newly on the road
  • 86 SE/GT "The Chameleon" - big plans!
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JazzMan
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Report this Post04-15-2008 01:44 PM Click Here to See the Profile for JazzManClick Here to Email JazzManSend a Private Message to JazzManDirect Link to This Post
The injectors receive power at all times that the key is in RUN, the ECM activates the injectors by supplying a ground path to complete the circuit. Injectors are normally closed and open when current is allowed to flow through them. They are fired in batches, the even number cylinders on one circuit and the odd numbers on the other circuit. Here's the basic circuit:



Check for power at the harness disconnect right next to the thermostat housing, if no power there pull the fuses and check for power on one leg in the fuse hole, if no power there check that all the fusible links are intact over by the junction block rearward of the battery.

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Gwain
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Report this Post04-15-2008 07:29 PM Click Here to See the Profile for GwainClick Here to visit Gwain's HomePageClick Here to Email GwainSend a Private Message to GwainDirect Link to This Post
A new twist for those of you still interested in following this diagnosis.

First off, did I say I "hate trouble shooting ignition problems"! This is my second car in 6 months trying to trouble shoot no start issues. I hope I'm getting better.

Anyway, I had another thought today after observing last night's test results on the fuel pump circuit. Tonight I redid the test light check at the ALCL on the fuel pump, but note this additional observation:
The car has been sitting 24 hours. Plugged in my test lamp, turned the key to "Run", light comes on for 2 seconds, fuel pump runs. Turn the key to "Start", starter cranks ... no test light!! A second time, turn the key to "Start", starter cranks ... no test light!! On the third try I get a test light under crank now, but also the oil pressure has pumped up! Wouldn't this suggest I'm really not getting EST pulses, it's just the oil pressure switch overriding the circuit?

When I did the test yesterday, it seemed I didn't get the test light with the first crank, but subsequently I did later. I didn't think anymore of it at the time. Last night thinking over the schematics and comments here it dawned on me that perhaps I wasn't getting EST pulses, but was seeing the results after the oil pressure switch took over.

Anyway, I gave it a shot of starter fluid and "no go"!!

I think tjm4fun had it right in the beginning - 2 problems - one fuel pressure, one ignition related.

At any rate, I'm pulling the distributor and replacing the exciter coil. It's only a $6 item. I've replaced the module already with a new AC as well as cleaning and reassembling the distributor completely. The only thing I didn't replace was the exciter coil (because it didn't look bad), but I've had the same symptom now from day one so it makes sense.

If I can get this thing running, then I'll tackle the fuel pump issue. I still need to check the fuel pressure regulator, but it seems pretty certain the pump, or at least the pump check valve may not be working.

Again, stay tuned

------------------
Marc in sunny Titusville, FL


  • 84 SE son's car, loaded
  • 85 Coup w/V6 transplant
  • 85 GT newly on the road
  • 86 SE/GT "The Chameleon" - big plans!
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tjm4fun
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Report this Post04-15-2008 07:46 PM Click Here to See the Profile for tjm4funClick Here to Email tjm4funSend a Private Message to tjm4funDirect Link to This Post
if you are seeing the tach move while cranking, the coil is probably good. no tach is module or pickup coil.
there is a tool called a noid light you can put on the injectors to see if they are firing.
tho if you don;t get anything with ether, you have no fire.
in the last 3 days I've had to help 4 people with no starts on their boats, spring is good for wtf happened? it ran when I put it away...
fuel+fire=run.
BTW, in your case, verify spark form the coil when cranking also, you could have a wiring issue from the module to the ecm, those connectors are getting flakey after 20 years.
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