Hello, new to the forum, 5 year Cadillac owner - 1995 Sedan Deville 4.9.

My Problem is that it starts as expected most of the time, but roughly 1 out of 20 tries yeilds a no start. It will turn over, but will not fire. I have checked the fuel pressure during a no start moment and there was none, but on occasion, the car will start after several failled attempts and then it obviously has fuel pressure (which I have verified by pressing the fuel rail do-dad). I have swapped relays in and out of the fuel pump relay location during a no start moment and no change. I have also removed all the relays while the car was starting fine and it still started - I have read that the fuel pump can recieve power through the oil sending unit. Can someone explain what the fuel pump relay really does since the car will start and run without the relay in place? There have been no codes to retrieve following a no start moment, but I will check for them again once another no start moment occurs.

No security warnings or lights.

I have always had trouble hearing the fuel pump running.

Releiving pressure at the fuel tank cap has made no differnce.

???? What am I missing ????

Any help would be appreciated,

 

I had the same problem when one day will driving home my car cut off and would not start again. I had to have it towed home and after much research I found my problem was with the module in the distribitor and after replacing it have not had problems since. By the way when it went out it also burnt out my altenator so I had it rebuilt.

 

Thanks for the reply Red1, but I have determined that it is not the module. The car had sat for months (since Feb) and I got around to trouble shooting the car which is very difficult to do since it's troubles are intermittant. At any rate, I changed the fuel filter and the car started working properly until last week. Once again, no-start issues.
Again, when a no-start occurs, there is no fuel pressure at the check valve (the tire-like valve near the fuel line) under the hood. If I put a test light on the connection for the fuel pump relay (with the relay removed) I get a momentary signal once thekey is turned to the “on” position. If I put the relay back in and disconnect the fuel filter NO fuel comes out during this momentary signal. If I remove the horn relay and the fuel pump relay and jumper some power from the horn terminals to the fuel pump, the pump will power up and push fuel freely. With the fuel filter re-connected and the jumper in place, the car will start and run properly. I can quickly remove the jumper and reinsert the fuel pump relay and the car runs just fine and will shut off with the key and even restart properly (sometimes) and then it’s back to not starting. The jumper trick will bring it back to life, but it may act normal for days/weeks or not start the next time it is attempted.

I am getting a related code for PO20 - "Open Fuel Pump Circuit" so I believe the fuel pump is fully operational, but it does not always get the power it needs to start and run. Since the jumper will cause the fuel pump to act as expected, I don't think there would be a short between the relay and the pump. My problem seems to be before the relay...

Any ideas on a cause for this? What's my next step?

 

I had an awesome 200 word solution to your problem but this stupid editor crashed when I tried to submit it. I'll try again when I get home, plus I should have you some schematics.

 

Thanks for the efforts Zynder, these editors can really bum you out. I look forward to your reply.

Last night when I got home the car started just fine keeping me from trouble shooting anything, but I decided to try and make it fail. I removed the pump relay and put my test light on it, checked for the momentary signal and got it and then checked for the ignition signal (power to the fuel pump at the relay socket - relay removed as I tried to start the car) and get this - it started and ran fine without the relay in place. It even reved up fine and I let it idle for several minutes. Just the other day when I removed the relay as the car was idling it sputtered a few times and died, but not last night, it started and ran without the relay in place.

I'm confused on this one!

 

Since this archaic message board software won't let me directly paste a picture in the message body (it has to be hosted on another server) I didn't post any schematics yet, sorry. I'll have to get an imgur or photobucket account set up. I'll try to explain it the best I can. If you have a schematic, please follow along.

There are only 5 devices in this system that can fail. They are: the PCM (computer), the pump, the pump relay, the oil pressure switch, & Fuse A1 (20A, engine compartment block, hot when in run & start). Since it works most of the time, we can rule out Fuse A1. When you jumped power to the pump from that other relay, it worked, so we can mostly rule it out too. You swapped relays with no change so apparently you have a good one as well. When you pull the relay while running, it continues to run, so the oil switch is closing when oil pressure is present. You'd also have a low pressure warning if it did not work properly. That only leaves the PCM but don't immediately jump on that as I'll explain further down. Also pin 4 has a grey wire that is the fuel pump priming connector. It isn't relevent for this problem.

The system works as follows: The computer sends a signal to the relay coil to close via pin 1 (dark green/white) and is grounded through pin 2 (black/white) under the drivers kick panel near the floor (ground G303). When the relay closes, pin 5 (pink/black) is connected to pin 3 (grey) to the pump which is grounded (black) behind the backseat (ground G401) on the right side of the X brace. That pink/black wire comes directly from Fuse A1. Fuse A1 also has a pink/black wire running to the oil pressure switch which closes when the engine has a high enough oil pressure. This goes from the switch through a grey wire that also hooks to the grey wire on relay pin 3. It also connects back to the PCM to let it know that it is running. This loop is how the power bypasses the relay which keeps the car running as you have tested. You asked why and the simple answer is because that is the way they wanted to do it. They wanted to make the engine shutdown in the event you dumped all your oil. If the engine is running but has no pressure, that switch opens which makes the PCM lose that feedback signal from the grey wire and the engine stalls. This is a good thing.

Troubleshooting: You believe that the system is screwed up somewhere before the relay. Sadly, there is nothing but that fuse and the PCM before it. So do this: with the car off & key in off position, remove the relay & verify Fuse A1 is not blown (it probably isn't). Acquire a multimeter that will read resistance and volts DC. Set it to resistance. Attach the black probe to a body ground. Take the red probe and touch relay socket pin 2 (blk/wht). You should have zero ohms resistance. This ground is VERY important. If it isn't zero or does some flaky stuff, we have found your problem. Now touch the red probe to relay socket pin 3 (gry). You will read some resistence here since that is going through the pump windings but it shouldn't be absurdly high, probably less than 1K ohm. If it is really high or showing OL then check the pump's ground G401 in the trunk. Touch the red probe to pin 1 (dk grn/wht). You MUST have OL/infinity/open whichever your meter displays. Touch pin 5 and verify OL as well. These wires carry voltage when operating so they absolutely cannot be grounded to the body in any way! Switch your meter to VDC & switch the key to run (don't start it!). Touch pin 5 again and verify 12V is present. Touch pin 1. There should be 12V here as well but it is possible that there is not. If not, remove Fuse A1 and have a buddy crank the car while you watch the meter. If 12V never shows itself, this is your problem. Reinstall Fuse A1. Switch key to off.

Remember I said that the PCM might be bad? No voltage on pin1 would point to this. I have worked on cars for quite awhile now (22 years!) and I have only seen one car actually have a bad PCM and it was through water damage. They typically outlast the car. What is possibly happening (some cars do this, some don't) is that the computer is not switching it on, on purpose! If it is missing an expected input then it may disable the outputs. What input would the computer need to decide it wants to turn the fuel pump on? Well you want to feed the engine fuel only while it is running or cranking. To figure out if the engine is running, the PCM is fed an RPM signal from the ignition module in the distibutor via a hall-effect or magnetic pickup. Red1 already mentioned the ignition module but you dismissed him quite quickly with no explanation. If all the meter readings are correct (and I bet they will be), I am telling you a second time, you have a bad ignition module! I had this exact same thing happen on my Seville 4.9L (random no starts, especially when hot, eventually died completely) and it turns out the module and both of it's connectors had been toasted! Normally one would think it would throw the code for cranking but no rpm signal, but mine never did.

The one time you got that P020 error code, I am gonna say was a fluke, possibly triggered while you were poking around in there. If it is real, it will come back every time you clear the computer and run it again. Also, understandably, you seem fixated on that momentary pulse. Don't be. It is a priming pulse and it is normal. It may not be strong enough to push fuel out of the line and is mainly to ensure the pick up and pump impeller chamber do not have air in them. When you switch the key to run but not start, it will prime, and upon not seeing any cranking, will turn off. This is why I told you pin 1 should have voltage but may not. When cranking & running, pin1 should always have voltage.

I hope this helps. This was long and in my own secret engineer language so if anything doesn't make sense, I'll try to clarify. These intermittant faults are some of the most annoying to troubleshoot so I wish you luck!

 

Excellent post Zynder, very well put. I have read through it and nothing jumps out as confusing to me, however I will have to apply your advice with my hands on the car which could lead to the said confusion. I have a multimeter and some time on tues to get down to business with it. Wish me luck and Thanks again!

And Red1, sorry for the ubrupt dismissal of your post, but I am a bit old-school (and/or a bit ignorant). My undertanding of an ignition module tells me that if it's bad then the car won't run no matter what. Apparently that is not the case and I do appreciate your response and everyones for that matter.

 

hello i have the same problem ! my car starts sometimes but then the engine dies while driving. there is still electricity because the dash and lights still work. i can not figure out why it is doing this please help