Crock Pot

We have a 1990 XLT Lariat with 4.9 EFI and 5-speed. We went thru issues with the coolant temp sensor awhile back and everything was fine after replacing it. Now we have an inconsistent situation where it will be hard to start, and run really rough with no power. It happens after a short stop / shut down at a store or post office and restarted while still hot. Not every time. The exhaust smells very rich while it is chugging. It will idle roughly, but has no power to pull away. Sometimes it will clear itself, and always cranks and runs fine after cooling off. It will have a engine light while it is acting up, but it goes off as soon as the condition clears. No codes are being stored, so nothing shows up when we check them. Idle speed is too low when warm, won't stay running.
Should I start with a TPS, MAP sensor, Intake temp sensor, O2 sensor or all 4? All new distributor cap, rotor, wires & plugs, so I don't think it is there. Rich smell tells me it is getting plenty fuel, so it about has to be a mixture / sensor condition, right?

Aktech

Pull codes and engine vac. Check timing and make sure all the plug wires are working. Timing may be hard to do if it ain`t running. We should know more when you get the codes.

Crock Pot

We replaced TPC and MAP sensors. The condition repeated again over the weekend. While the condition is occurring, the CEL is on. Then it will clear up, the light goes off, and no codes are stored so there's nothing to retrieve. The fuel pump in the tank is audible when it is acting up. I'm beginning to think it is a fuel pressure sensor issue up at the fuel rail.

Just call me Sean

iTrader: (1)

If the light came on there is a code in there. If not, your computer isn't keeping memory.

oldmanb777

If the light came on there should be a code. But OBD1 isn't the best at keeping codes. Look for the fuel pressure regulator, it's vacuum operated, and if the diaphragm in it leaks it will run rich at times,maybe not all the time. Similar to what you describe. Not an uncommon problem, and won't store codes. The only thing the computer will know is a rich condition, which in your case is only temporary. Most I have seen suck a small amount of fuel past the diaphragm, and into the vacuum system. That causes all kind of odd running issues.

joshtowal

I'd put my money on a weak fuel pump. If the fuel pressure at the fuel rail is low, the heat from the engine will cause it to evaporate in the lines by then engine and cause a vapor lock. As long as the truck is running, the fuel isn't sitting by the engine heat long because it's being continuously circulated. Once you turn it off, it's just sitting there by the engine getting heated up. After a while, when everything cools off, the fuel condenses again and you can start it right up normally. Check your fuel pressure before you go guessing at sensors.

Crock Pot

We replaced fuel pumps over the weekend. Yesterday, the condition repeated itself when hot. On the way home, I stopped and popped the hood to look around, didn't see anything loose. I didn't close the hood fully, but left it held down by the hand latch since I wasn't going fast or far. After getting rolling, the condition cleared up in bout 100 yards. I believe it is vapor locking. I didn't think it was possible with a fuel injected system, but this design has so much stuff jammed in the right side of the engine. I think we will try insulating the exhaust, and maybe taking the EGR system out of circulation.
Thanks to all for their help

Bullfish317

I have had this problem recently as well, I replaced the throttle body gasket and cleaned the throttle body when I had it off (there was a lot of gunk in there). I also cleaned the contacts on all of the electrical components. Now the engine starts right up when cold. After I get back in and start the engine (warm) it has a rough idle until I give it a little gas, then it is a champ.

Hope this helps!!! Good Luck!!

candyspaw

While in the process of figuring out a lot of problems with my F-150 '92 4.9l-6, I discovered that my restart hot problem was due to flooding. I could get it restarted by flooring the gas while cranking then goosing to keep it running. Lots of raw gas and smoke. Theorizing stuck injectors(it started great every time after a few hours and seemed to run great), I gave it a diet of Seafoam. Ran the **** out of it and now the hot starting issue has gone away. Hope that helps.

Crock Pot

That's helpful. I have run some Seafoam through it. Replacing fuel pumps might be the answer. In other threads I have found that there was a vapor lock problem with the early generations of fuel injection because the fuel rail is located right above the intake runners and exhaust manifold (both intake and exhaust on the same side of the heads). When sitting still, the heat just convects up across the fuel rail. It will start sputtering if stuck in traffic on a hot day, and won't correct itself until it cools a bit. Popping the hood to let fresh air in while driving (below 40 mph!) will usually correct it. I understand that Ford added a small fan under the hood as an interim solution, then increased the fuel pump pressure and decreased the size of the fuel injectors in later models, which raised the boiling point of the fuel in the rail.