Hard Start/ Rough Idle on Start

[Kirby N.]

Hey fellas-
I am working on my 1991 Ranger with a 4.0 and a manual. It runs really well and has plenty of power. The only thing is it is hard to start cold or hot. It requires some throttle and then it idles rough for a few seconds until I rev it up and clean it out. It is almost like having a carb again! If I dont give it throttle right after it starts it will run really rough and die, then start the same way again. It does start easier if I have ran it recently, but it does take more cranking and throttle even if the engine is warm. There is no CE light.

Here are some of the things I have done to try to fix it (with some research):
New Carter Fuel pump
New fuel filler neck (old one was broke and I thought it wasn't maintaining pressure in the tank)
New Spark plugs
New IAC sensor

I have a new MAF sensor to put in it but I havent put it in (thought I would see if anyone concurs before I put another $80 part in it that might not work)

 

[Kirby N.]

So does anyone have any ideas I should try other then a MAF? Does the MAF sound logical? Thanks in advance

 

[Runnin'OnEmpty]

Since it runs well and has plenty of power,
I doubt if the trouble is in the MAF.

Have you cleaned the throttle plate? Sometimes
carbon will build up on the throttle body/plate
and affect the idle.

 

[Kirby N.]

Is the throttle body between the intake plenum and the rubber tube coming out of the MAF?

 

[Runnin'OnEmpty]

Is the throttle body between the intake plenum and the rubber tube coming out of the MAF?

Yes. It's real easy to remove the air tube and access
the throttle plate. If it's carboned up, it'll be where the
throttle plate meets with the throttle body. Clean the
carbon out and see if the idle improves...

 

[Kirby N.]

Ok. I have been in and out of town, so I havent had time to clean the throttle body. I have some time tomorrow and I plan to do it if the weather is nice. Another concern though-

I have been out of town for 17 days and I havent driven my truck since then. When I started it today, it took easily 3 minutes of solid cranking with throttle before it would sputter to life, then it ran rough for a few minutes until it got going. It started fine the rest of the day. It makes me think fuel? Any ideas?

Thanks in advance!

 

[Kirby N.]

Ok so I got a fuel pressure gauge. The fuel pressure runs at 32 lbs if I unplug the regulator vaccuum it goes to 40 lbs. when I turn it off it goes to 38 lbs and maintains for at least 10 minutes. That all sounds proper to me. Anyone disagree?

 

[Kirby N.]

I am going to leave it for 30 minutes and check back. This has my head scratching. I noticed that it seems to start better if I cycle the key 15 or 20 times before I start it, so I thought it must be fuel system related.

 

[2000StreetRod]

barely meets spec

Ok so I got a fuel pressure gauge. The fuel pressure runs at 32 lbs if I unplug the regulator vaccuum it goes to 40 lbs. when I turn it off it goes to 38 lbs and maintains for at least 10 minutes. That all sounds proper to me. Anyone disagree?

My Haynes Manual specifies the fuel pressure with hose attached at 30 to 45 psi and with hose detached at 40 to 50 psi. After ignition off for 5 minutes should be 30 to 65 psi. Your pressure meets the specification but just barely. However, there should be adequate pressure for the engine to reliably start and idle. There shouldn't be any increase in fuel pressure after the third ignition cycle. Your engine may be running lean at start up. That could be due to an intake manifold leak or a defective engine coolant temperature sensor. Have you examined all of the intake hoses for cracks or being loose?

 

[Kirby N.]

Ok, so the fuel pressure didn't last 30 minutes. At 30 minutes it was at 10 lbs. Don't know if thats enough or not?

I cleaned the throttle body, cleaned the MAF, and checked all the intake between the airfilter and the intake. I havent ran it yet, but I didn't see anything out of the norm, so I expect the same in the morning. Any other ideas?

Also, sometimes there is a little puff when it starts hard and finally starts.

 

[2000StreetRod]

fuel pressure regulator?

When you removed the vacuum hose from the fuel pressure regulator (FPR) did you notice any fuel in the hose? If so, it may have a ruptured diaphragm. That would explain the rapid loss of fuel pressure after ignition off. If you have access to a hand vacuum pump it is easy to determine if the FPR will hold vacuum.

 

[Kirby N.]

I don't have access to a hand pump and I didn't notice any fuel in the vacuum line. I will check again though.

 

[Kirby N.]

So I am still messing with the dumb thing. No fuel in the Vaccuum line. She starts a little better and seems to run smoother, but still starts hard. Also noticed since I cleaned the intake etc.- it idles at 15K RPM after it warms up, which is different than before. (about 5k rpm higher) might need to reset the computer??

How long is it suppossed to hold fuel pressure? If I am loosing it in 30 minutes, is that causing my hard start issue? Could that be an injector that isn't staying shut? If so, how do I figure out which injector is my problem (short of replacing them 1 by one)

 

[2000StreetRod]

fast idle & pressure loss

. . . Also noticed since I cleaned the intake etc.- it idles at 15K RPM after it warms up, which is different than before. (about 5k rpm higher) might need to reset the computer??

I assume that you meant 1500 rpm after warm up and 500 rpm higher than before. I suspect that you either have an intake leak, the IAC valve is stuck open or that the throttle plate is not closing. You might try disconnecting the throttle cables to see if the idle decreases. If it does then you don't have any slack in the cables. Have you ever adjusted the throttle plate stop screw at the throttle body? In your first post you stated you had replaced the IAC sensor. Did you mean the IAT sensor or the IAC valve?

How long is it suppossed to hold fuel pressure? If I am loosing it in 30 minutes, is that causing my hard start issue? Could that be an injector that isn't staying shut? If so, how do I figure out which injector is my problem (short of replacing them 1 by one)

I suggest correcting the fast idle before working on the fuel pressure issues. In an earlier post you stated that the fuel pressure was 38 psi after ignition off for 10 minutes. Does it drop to 0 psi after 30 minutes? If so, try cycling the ignition on/off three times when restarting after 30 minutes before cranking the starter. That should bring the pressure up to normal and make starting easier. If it doesn't then the fuel pump is weak, the fuel flow is restricted, or the fuel pressure regulator is defective.

 

[Kirby N.]

Thank you that helps a bunch. I replaced the IAC valve. I have never messed with the throttle stop screw, other than opening up the throttle body, taking it off and cleaning it out. I will take a look at it though. I noticed that screw. I must have changed something because it wasnt doing that before I messed with it.

it dropped to 10 psi after 30 minutes, not quite to 0. I have replaced the fuel pump but I think it was a lifetime one from napa and I could replace it again at no cost. The fuel filter has been changed but I could do it again easy enough- either way it sounds like the regulator might be the culprit. Injectors bleeding the pressure off aren't likely in your opinion?

Thanks for all the help!

Kirby

 

[2000StreetRod]

wait on the fuel pump

10 psi should be enough for the engine to start especially since the pressure should increase as soon as the ignition is switched on. My Volvo turbo wagon started (but ran poorly) on only 0.5 psi. If you have a leaking fuel injector it would leak all of the time there is pressure and the spark plug in the associated cylinder should be wet.

Have you watched the fuel pressure rise each time the ignition is switched on and off? It should rise to normal in three ignition cycles. It should also be normal when the starter motor is cranking the engine. The PCM detects the crankshaft rotation via the crankshaft position sensor and enables the fuel pump during ignition switch start.

Fix the fast idle first. An unmetered intake leak causes a lean condition which results in hard starting and rough idle when the engine is cold.

 

[Kirby N.]

So I put on a new fpr and unbolted the battery for a while (swapped out the optima for a cheapy- needed the optima in another vehicle). If I start it without touching the throttle, it will crank a bit, then start and idles at 1k rpm pretty normal for about 4 seconds then dies. If I use some throttle it will start ok, still cranks a while and still sputters a bit when cold. I am out of ideas.

 

[2000StreetRod]

ect sensor?

Did you ever check the engine coolant temperature sensor? Wire to sensor could be broken, connector to sensor could be loose, sensor could be defective.

 

[Kirby N.]

Is that the one right near the fpr? The guage works ok, so I thought I ruled it out? Tomorrow is snow, so maybe I can check friday or saturday.

 

[2000StreetRod]

two sensors?

I don't know about your Ranger but on my Explorer SOHC V6 there is an engine temperature sender that drives the temperature gauge on the instrument panel and an engine coolant temperature sensor that provides data to the PCM. The first one has no effect on engine operation. During normal operation the PCM enriches the air/fuel ratio when the engine is cold. If the sensor providing it data isn't functional then the engine may be hard to start and idle poorly until it gets warmed up. The sensor resistance decreases as the temperature increases. If the wire to the sensor is shorted to ground the PCM will think the engine is hot.