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TxBuicks

1991 Roadmaster Won't Start

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This weekend my 1991 Roadmaster (5.0 Liter, 305 CI V-8) got to where it won't start. I need some expert opinions as to where to start looking. Here's what happened. On Saturday I changed the oil and filter. Nothing new there, except I may have overfilled it by a quart or so. But after changing the oil, the next 5 times I started it, the car would fire off after a few engine revolutions like normal, run for 5 seconds and quit. But it would start right up immediately after that and run just fine until I would kill the engine again after I got to my destination. After several occurances where it would start, then die after 5 seconds, it would not restart. Engine turns over like it wants to start (perhaps too fast?) , but nothing. I have verified that the throttle body is getting fuel so it must be electrical. Another thing I thought was weird is that my A/C stopped working. Blows air but not cold. I think changing the oil and the A/C is just coincidental. Perhaps too coincidental. How could all of this be related?

Anyway, before I start replacing all the ignition components I thought I would get a few opinions from the Buick experts on this forum first. I'm thinking it is the electronic ignition module inside the distributor cap, you know the distributor that they placed in the very rear of the engine up against the firewall, where you have to be 7 feet tall to reach it and have tiny hands to work on it? But I replaced that thing last year. Any other ideas?

Edited by TxBuicks (see edit history)

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Roy

I seem to recall that some GM cars had a "kill engine" capability built

into the circuitry/ECU .....

Such that, if low or no oil pressure indication came from the sending unit

the default was to kill the ignition or stop fuel flow or both.

Shop manual might shed some light there....

m

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RM75, that's what I thought about when I over-filled the oil, but the shop manual says nothing about it. Anyway, I may have been mistaken about the fuel. Last night, in the 15 minutes of daylight I had left after work (thank you Daylight Savings), I managed to pour some gas into the throttle body and it started and seemed to run alright until the gas ran out. I only tried it twice due to a low battery, due to turning the engine over and over trying to get it to start over the weekend. So I charged it up last night and will investigate it further tonight. But now I am somewhat relieved to think that it may not be electrical after all. In my opinion, fuel problems are much easier to diagnose and repair than electrical problems. Although the fuel pump is in the gas tank.......could I be so lucky to find out it is only an in-line filter?

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There should be a fuel filter on the left side just head of the rear axle. I have never had one plug. They do sometimes rust out if a bit of water collects in the bottom corner from the way they are angled.

Bernie

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Replace your fuel filter, as said before. Also turn ignition on to "Run" and listen around the fuel tank for the fuel pump. Funny noise? No noise? They have a way of failing all at once (my '96 Impala) or gradually (my '94 LeSabre).

It also may be a fuel-pressure regulator. I would try replacing the filter and regulator first. They are cheaper and easier, and on an older car not a bad idea anyway.

There comes a point, though, when guessing with new parts gets more expensive than a mechanic who can diagnose more accurately...

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If the injectors don't get a full (approx) 55psi fuel pressure, they will NOT fire--period. This means a fuel pressure check. Unlike a carburetor, the presence of fuel at the TBI unit is NOT an indicator of something good.

The fuel pumps usually whine when they are getting on their last legs, which can be aided by a fuel filter that clogs over time. Usually, though, with a clogged fuel filter and/or weak fuel pressure "build", the result is extended crank time as the fuel pressure builds until it's high enough for the fuel injectors to fire.

The workings of the security system will not be in the chassis manual, but the body service manual. Usually, though, it's a full fuel pump cut-off such that ONLY cranking happens, not a partial-start situation.

There is an oil pressure switch in the electrical system to prevent the fuel pump from running in the event of a sudden oil pressure loss. On a "start" situation, there is a timer which starts when the key is turned on, to turn on the fuel pump in anticipation of a "start" activity, but will turn off the fuel pump after about 45 seconds in "ON" without a "start" situation following.

Sometimes, the oil pressure sender on the upper center rear of the motor (where they normally are on Chevy small block V-8s, back then) is a three-wire switch, with the extra wire being the trigger wire for the fuel pump circuit. In other situations, there is a separate switch which screws in just above the oil filter, in the side of the engine block. That one is a two-wire "continuity" switch for the fuel pump circuit. In some other situations, they moved the whole thing to the side of the motor, above the oil filter.

Another possibility could be a fuel pump relay that's getting weak. It might well be on the "bracket" which holds multiple Bosch relays on the firewall. What I term "universal Bosch relays" as the same relay part number is used in different circuits. There's a relay for the a/c system on there, too.

My suspicion is that it's more of an electrical supply issue than a pump failure per se or a security system issue.

Keep us posted on how things progress.

NTX5467

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Problem solved. I drove it to work this morning. It was the fuel filter mentioned by 60FlatTop above. $11 and 30 minutes of my time fixed it. I believe the old filter was original, and 185,000 miles of use clogged it. I could not even blow air through it. It is amazing any fuel made it through. It seems so trivial now, asking the Buick experts for advice on a clogged fuel filter. But I have learned a lot about early fuel injection systems, of which I have not had experience before. I have not seen a clogged fuel filter act that way. My experience with carbureted engines is that you will gradually see a performance decrease due to a clogged filter, not all or nothing as was the case here.

NTX5467 was wise on several points. In my initial diagnosis I pulled the fule line at the throttle body and it squirted gas so I assumed it was an electrical problem. But once I poured gas into the engine, it fired off and ran until the gas ran out. NTX5467 was right, the injectors were not firing due to lack of pressure, even though I had gas up to the throttle body. Also, he mentioned a clogged filter would take more cranking to start, needing extended time to build up pressure. I noticed that immediately. After I installed the new filter, the engine fired off immediately. For the past several months, I had noticed the engine cranking more and more before it would start, but I assumed it was due to lower compression because of the high mileage. And, since it would start, I didn't give it much thought. But the clogged filter was causing that, too.

Anyway, thanks for all the advice. Perhaps me posting this will be a learning tool for anyone else experiencing the same symptoms.

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A carburetted engine basically carries a forgiving little pot of fuel on top of the engine. It all works under natural pressures and engine differentials.

Fuel injected cars only have the line volume and pump pressure. You don't have a chance if they are not up to snuff.

Always remember to carry at least a half a tank of fuel to cool that little high speed pump. And if you R&R the fuel tank run a ground wire.

Bernie

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YES, many people forget that any electric motor will generate some heat as it runs, especially a pressure pump. The fuel in the tank is the "heat sink" which absorbs heat from the pump and disperses it in the liquid fuel to keep the pump "cool". If you think about how much volume a gallon jug holds, then how deep the fuel in the tank will be at the 2-3 gallon level (when the low fuel warnings usually start), then you realize that not much fuel is around the pump housing at that time.

In many vehicles, there's a plastic baffle which will keep fuel around the pump's intake strainer.

I'm glad it was a simple fix! We used to recommend changing the fuel filter at 30K mile intervals, although GM had no such recommendation in their maintenance schedules at those earlier times (1990s+). When a tech would remove a clogged fuel filter, only fuel would run out, which raised the question of just what clogged it up? Especially with the fuel strainer in the fuel tank still there!

Take care,

NTX5467

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Wait. The story continues. After changing the fuel filter everything ran great, until Saturday. I had driven the car everyday since I replaced the fuel filter and no indication of any problems until Friday night. Car started lurching, loss of power, then nothing. Wouldn't start. I was 25 miles from home. Called my wife to come rescue me, bought another filter, replaced it in the parking lot, but still nothing. Pouring gas in the throttle body still proved it was not an electrical problem because it ran until the gas was gone. Now I suspect a fuel pump. Had it towed home. Couldn't do anything to it Sunday except get it jacked up and gas siphoned out of the tank. I will pull the tank to get to the pump in the next few days. I know the original fuel filter was bad becasue I couldn't even blow through it when I took it off, but I guess the pump was going bad at the same time. The new filter just bought it a few more days. The good news is that a new pump is under $100 and I feel comfortable replacing it myself. It should be on the road again shortly.

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Is it so hard to put a drain plug on the gas tank?

LOL... Imagine if they did, not many would find their tank of gas in the morning where they left it the night before.

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You're probably right, John. I guess I don't think like a criminal. The problem is that the filler tube goes into the side of the tank about half way down, so the best I can do as far as siphoning is half a tank. I took the line loose at the filter and turned the key on thinking the pump would come on and pump it out. But, no, nothing from the pump. At least that tells me the pump is definately bad. I was thinking maybe it still pumped but not enough pressure. I will half to wrestle with half a tank, I guess. Hope to finish up this weekend.

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When ever I use the drain plug on my 56 I always find myself thinking, this is stupid. The fumes from the gas are pretty much overwhelming while I'm down there draining the tank. I even used a fan once to blow air under the car away from me but it wasn't as effective as I though it would be. Wrestling with the tank is pretty much the only way to go.

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This procedure that I used on my 76 Olds may have a variation that may work in your situation: blow low pressure air into the return hose at the fuel pump catch from the fuel line...gas cap on of course.

Willie

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Be sure to look at the wiring on the gauge sending unit. When the pump is pumping against a somewhat clogged fuel filter, it pulls more amps and can melt the insulation on the wiring. Two ways to fix that . . . a new sending unit (without pump) or an aftermarket wiring pigtail. This would be the wiring harness on the sending unit itself, between the pump and the connector where it connects to the chassis wiring harness.

Keep us posted . . .

NTX5467

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Fuel pump fixed the problem. Had it back together on Friday and been driving it all weekend. Thanks for all the encouragement and thoughts. $100 for parts, and about 10 hours of my free labor. Of course, I had to clean and repaint everything before I put it back together. Many more jobs like this and I'll have to get a lift.

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