hard starting when hot, bad idle, flooding, sputters

[lonestar]

I am having a weird problem. I first experienced about 2 months ago, I was driving on the freeway, got off, and while sitting at the red light it started stumbling. Giving it gas would not do anything, it just ran like crap. It would die, start it and it would not run right. Finally I floored and it backfired through the intake, coughed and sputerred and cleared up for awhile. Did it once more down the road and was fine until yesterday.

I stopped for a minute, then I tried to restart it and sputterer like it was flooding or out of time, but no black smoke. Finally got it started and ran fine. Today I left it running, got out to talk for a minute and it started to stumble and sputter, tried to give gas and it died. Restarted and revved it a couple of times and it cleared up. Went to a store, went in for about 20 min, came out and ran little rough for awhile. Drove home on freeway, went inside for about 10-15 minutes, came back out and it would not start, coughed, sputter backfired. Took wifes truck out for about an hour, came home and it started fine.

It's weird, can't tell if it's fuel or spark. Seems to me like it is flooding sitting at idle like a leaky injector or regulator.

 

[Alec]

It does initially seem like the FPR. Could be sensor related, but does seems like the regulator.

 

[lonestar]

I guess I need to put a pressure guage on it. Do you think a faulty regulator would lead to flooding and sputtering? Possibly increased pressure causing injectors to inject too much fuel. It just seems weird too me.

Runs fine, and then starts to sputter and die when left idling for 2 min or so at a light. Seems to clear up if you can keep it running and drive for awhile. Also happend a couple of times after hot start.

 

[Rhett]

If the FPR is bad, it can cause flooding and an over-rich condition, leading to the poor running you're getting. Also your truck's O2 sensor could be fouled because of the bad FPR sending too much fuel through. This can explain why it's doing it when the truck is heated up.
I'd check the pressure and if the FPR is bad, I would replace the O2 sensor while I was at it (your 91 has 1 O2 sensor, lucky you, it's cheaper than buying 2 or more!).

 

[lonestar]

If it was the FPR, wouldn't it have the same problems whether it was cold or hot, and wouldn't it blow black smoke if it was flooding.

What do you think about a bad O2 or EGR?

The hard part to understand why it would only happen when hot and left idling for a couple of minutes, or hot startup after a couple of minutes. Short stops or a quick restart does not cause any problems.

 

[Rhett]

Your 91 doesn't have EGR, unless it was added later (which is unlikely).

Like I said it could be a bad O2, in fact it probably is a bad O2 if I had to make a judgment from here.

When the truck is cold, the computer knows this and sets the FPR and injectors to let the engine run richer until the engine warms up. So the FPR will let more fuel into the rail when the engine is cold to warm the engine faster. Then after the engine is warmed, the computer will direct the FPR and injectors to lean out the mixture (let less fuel in). If your FPR is stuck then it's never leaning out, always letting more fuel in. This is most apparent by bad mileage and sometimes a poor idle quality because of the flooding. The flooding can ruin an O2 sensor pretty fast too.

The O2 sensor requires a certain temperature to turn on.If it never reaches that temp then it is OFF, DEACTIVATED. The computer is programmed to ignore any O2 signals until it heats up. That's why you can take short drives and restarts and it's not a problem. Don't just replace the O2 without making darn sure the FPR isn't bad as well...otherwise you'll be wasting your money because a bad FPR will destroy a new O2 fast.

Check to see if there is any fuel in the vacuum line that connects the FPR with the upper intake manifold. It should be dry. Also check fuel pressure at the rail.

 

[lonestar]

Thanks, I'm going to check the pressure today. There is no fuel in vacuum lines. I didn't know the pressure also varied during warm-up.

I can drive it all-day long with no problem, I just can't stop for longer than 10 minutes and restart without problems. It's like the timing is messed up until the computer realizes the engine is still hot. My gas mileage really has not changed, still 17-19 MPG. It may be running lean also, I don't see any black smoke.

I have now also began experiance other start-up probs, but I think they are starter related.

 

[410Fortune]

One important thing, does it give a CHECK ENGINE LIGHT when this problem occurs?

If it does then pull the codes (free at Autozone)

After you check the fuel pressure, 02 sensors, etc.....dont count out the engine's computer itself....

I have seen 4 go bad to date.

 

[lonestar]

Nope, no CEL, but I will see if there are any stored codes.

 

[Rhett]

Check the connections to the computer like 410 said.
The comp is in the passenger side kickpanel (as you're sitting in the pass side seat, it's by your right foot). If the computer is bad it won't matter if it's connected good or not though.

Also, is the fuel pump ok in the truck?

You probably checked these already, but are the plugs and wires ok? Air filter? Battery cables?

 

[lonestar]

Checked the pressure. It seems low, starving for fuel. When first start it reads about 35 psi, then as it idles it slowly drops down to about 20 psi, and begins to idle rough, but does not sputter of bog. Also I tried pulling the vacuum hose of the regulator, but nothing changed, maybe the instant I pulled it off, but it returned. Drove it around a little, and tried to get it to sputter, stopped, restarted, but it never did it. Pulled the pump fuse, when the pressure got down to about 15-18 psi, it started giving me the same symptoms.

According to Haynes.
Fuel Pressure
30-45 with vacuum hose attached
40-50 with vacuum hose removed

Is there anyway to check the fuel line pressure to the regulator to make sure pump is good. According to receipts, previous owner replaced pump, have to look up date, but I think it has been within 3-4 years.

Replace air filter, fuel filter about 3 months ago.

BTY, it passed emissions test with flying colors about 2 months ago.

 

[lonestar]

Is there any way to check the output pressure of the pump itself? Is there a schrader valvel betweent the pump and regulator?

Also, when I first attached guage it read zero, then each time I primed it by turning the key to run pos, it went up about 5-10 psi until it reached 35 psi. Once it started, the pressure slowly bled down to 20 psi in about 5-10 min.

When I replaced the fuel filter it was completely clogged, would not run longer than 1-2 minute before dying, wonder if I cooked my pump. I'm just confused why a regulator would read low, unless the input pressure was low.

 

[MrShorty]

I already responded in the other thread, but I'll add a couple of things here.
The Schrader valve on the fuel rail is already between the fuel pump and the regulator. The fuel supply circuit is fuel pump - lines - filter - lines - fuel rail (injectors) - regulator (pressure steps down to tank pressure here)- fuel tank.
A regulator (technically these are "back pressure regulators", if that means anything to you) would read low if the input pressure was low, that is correct. The input pressure could be low because of a bad fuel pump, a clogged filter (already replaced one) or clogged/pinched/restricted supply lines. A regulator would also be low if the spring in the regulator were weak, or there were a leak in the diaphragm.

 

[lonestar]

The fuel supply circuit is fuel pump - lines - filter - lines - fuel rail (injectors) - regulator (pressure steps down to tank pressure here)- fuel tank.

I don't think so. The regulator regulates the pressure to the injectors. The injectors need a regulated pressure in order to consistantly meter the fuel. If not, then the injectors would be getting the full output pressure of the pump.

I think its more like this
Tank-pump-filter-regulator-injectors-tank

 

[MrShorty]

A backpressure regulator works like this:
Below the designed pressure, the regulator is closed. As the pump pumps, the pressure on the upstream side increases until the desired pressure is reached. At this point, the regulator opens, allowing excess flow to return to the tank. A properly functioning backpressure regulator maintains a constant pressure on the upstream side by varying the amount the diaphragm is open.

 

[lonestar]

Yeah, your right, I just looked in book, It bleeds off the excess pressure with return line. I was thinking more of like a air compressor which regulates down stream pressure, I think the newer X's work like that, with the regulator in the tank and no return line.

Anyhow, I just did another check to see if the regualtor was stuck open. I removed the return line, and nothing, it seems the pump is not building up enough pressure.