Solved [FIXED] Running extremely rough/rich on cold starts (w/ video)

[TorontoGuy]

Hey guys, an issue sudddenly popped up on my 91 Explorer X, 5-speed.

On cold starts (like after sitting over night) it runs extremely rough and seems like a pretty bad misfire. As you can see in the video the entire truck shakes and the exhaust seems like it sputters. There's also a very heavy smell of fuel around the truck - which there never was before.

When you try and drive the truck, it wants to die as soon as you begin letting out the clutch.

In the video is the worst it has been yet. Previously, I have been able to drive it and once it gets up to operating temperature it isn't as bad - but still not right. I have replaced the throttle position sensor as it was throwing a TPS code. I have also cleaned the MAF and replaced the air filter. The plugs and wires are about a year old with roughly 10,000 miles on them.

I'd just like to hear if anybody has any starting points or anything I could check.



Thanks!

 

[vq5speed]

Have you check all the vacuum lines for leaks. Check the vacuum line going to the Fuel Pressure Regulator. If there is fuel in the hose could be a bad FPR.

 

[cogs]

i was thinking vaccuum too, lol, around the octopus, hope it's an easy fix.

 

[the_don]

engine temp sensor or air temp sensor, if the above two post do not help.

 

[TorontoGuy]

Thanks for the help guys. I'll start tracing those vacuum lines.

How would an engine/air temp sensor cause my issue? (Not to sound ignorant, I'm just genuinely curious)

 

[natenkiki2004]

My understanding with the ECT & IAT temp sensors is that if they fail, they can make the computer think your engine, and the air it sucks in, is always warm. This means you're not getting enough fuel when cold, causing you to run a lean condition. Cool air is more dense and contains more oxygen so it's important to know the temperature of the air coming in (IAT) so the computer knows to shoot more or less fuel into the engine to compensate. Same for the ECT, a warm engine doesn't require as much fuel since it atomizes and ignites easier.

You might say "wouldn't the O2 sensor catch this lean condition?". You'd be right, except the O2 sensor takes time to warm up and until it's up to temp, the computer ignores it's input (open loop, I think it's called).

These are both cheap fixes that take 1 plug, an adjustable wrench and under a minute to replace each.

Both of my sensors were faulty but they didn't cause this extreme of an issue so I'm a bit leery of it fixing this problem but it's a cheap and easy place to start and they're known to go bad.

 

[turboranger91]

Something else to check... A bad MAF will cause a craptastic idle as well.

 

[TorontoGuy]

So I did a little deeper digging and tested the fuel pressure. Here is what I found.

Pressure when priming (Key On Engine Off) - 42psi
Pressure while running - 35psi

However, while running the fuel pressure spiked to 80-100 psi at times and would flutter in that range.

While holding the pressure release button on the pressure tester, it took over 1 minute to fill up 500ml bottle and I also found that it was mostly air coming out the end of the tube.

The fuel filter was replaced about 8 months ago.

Do these symptoms sound like a regulator issue or pump issue?

Thanks!!

 

[MrShorty]

Thinking about the way these are designed, it seems unlikely to me that high fuel pressure would be caused by the pump. The pump should be capable of generating that much pressure, it is the responsibility of the regulator/fuel return lines to allow the excess fuel to return to the tank. Off the top of my head, I see only two possible causes for high fuel pressure -- a stuck closed fuel pressure regulator, or some kind of kink/blockage/restriction in the fuel return line. Since it seems to come and go, I would be more likely to suspect a bad regulator.

 

[TorontoGuy]

Thanks for the reply MrShorty. Do you have any idea why it was mostly air coming out of the fuel pressure tester? It almost seemed like the fuel pump was pumping air.

 

[TorontoGuy]

It's fixed!

It was the fuel pressure regulator!

I also replaced the throttle position sensor, and the 02 sensor. Cleaned the MAF and the idle control valve and the engine has never run smoother!

Thanks for the help guys, hopefully this info will lead somebody in the right direction in the future.

Cheers.

 

[swshawaii]

Since it seems to come and go, I would be more likely to suspect a bad regulator.

The bottom line. Great call!

 

[the_don]

Thank you for the update.

 

[natenkiki2004]

Looking back, that makes a lot of sense. Good job and thanks for sharing.