Just purchased 91 F150 XL 5.0 V8. Engine sounds good but knew it had some issues when I got it. I’m a shop carpenter and pretty mechanically inclined, just don’t know much about auto mechanics. Thought I would “get involved” as my friend says. Fixed a bunch of stuff related to a non functioning temp gauge on the dash and now it won’t start.

The first thing I wanted to fix was the temperature sending unit. While I was doing that I noticed the idle air control looked new, but wasn’t hooked up. When I plugged it in the truck would idle high. Unplugged ran fine. Then noticed wires on the throttle position sensor were stripped and crossed. So I took off the throttle body and replaced the TPS, cleaned the throttle body with a Teflon safe throttle body cleaner, replaced the engine coolant sensor, and went a head and replaced the thermostat.

When I finally got it back together I went to start it up and it’s pumping fuel, starter is turning, but not getting ignition. The computer has sent two different codes 31 and 84. I also wondered when trying to get the thermostat housing off if I had maybe jostled the ignition module and damaged it.

Could stuck EGR or bad EGR solenoid prevent spark? Could I have damaged the ignition module on the distributor when loosening the thermostat housing bolts? Any type of reset with battery off for 2 days? It’s just weird cause prior to “fixing things” it fired right up every time.

 

As for the idle air control valve it sounds like you have intake vacuum leaks letting air in.
I get the idea that you did not change the sensor for the temp gauge but you changed the sensor for the computer.
I do not think that you damaged the ICM by changing the thermostat.
Also you should not clean the throttle body.

You need to check for spark and fuel pressure.
Is the selected fuel pump running all the time the key is in the run position with the engine not running?

 

The fuel pump sounds like it runs for one second just prior to turning it to start. I smell gas getting to the engine.

The temp sensor I changed is located right above the thermostat housing on the engine side and has a hose that goes to the throttle body right above it.

Tested a wire from distributor cap to spark plug with an in line tester and it showed no spark.

Replaced the starter solenoid or relay, still learning names. The thing with 3 prongs on the side wall right behind the battery.

Still no spark.

 

So I just tried the inline tester on the wire running from center of the distributor to the ignition coil and got spark. Hooked everything back up and now I get combustion but engine doesn’t want to run. Dies instantly. Don’t know if it’s flooded from all my previous starting attempts or what.

 

What you replaced was the sensor for the computer. The sending unit for the gauge is on the left (driver’s side) front of the intake manifold. It is a one wire sender.

 

Thanks so much for your help. I will replace the sending unit. Now that the truck ignited but quickly dies I’m wondering if I messed up by cleaning the throttle body. I removed the build up with throttle body cleaner and some industrial q tips. One thing I did notice after removing all the build up is that the butter fly valves have space around them when fully closed. Guessing that’s not supposed to be like that.
If one has cleaned a throttle body that shouldn’t have been is it best just to get a new one?

 

There should be some space between the butterflies and the bore. The stop screw is set to keep them from completely closing and binding in the bores. They are not available new except from BBK which are very expensive and I have had nothing but problems with them. I bought one to replace the worn out throttle body on my pickup. Within a year it had become so stiff that the cruise control cable would pop off trying to open the throttle and made it almost so I could not drive the truck. I sent it back to them. They went through it to see why, “fixed” it and sent it back to me but it still wasn’t right. I found an OEM one at a junkyard and have been using that one ever since.

It is possible that the plugs are soaked with gas. Most likely though is that you missed something during reassembly. Double check your vacuum hoses and electrical connections.

 

Thanks so much for your help. This sight has been awesome. I think my lack of experience and knowledge may have caused a fatal mistake however, or atleast it’s time for an expert to do the diagnosis.

I was under the truck trying to find the fuel pump to see if that might be part of my starting/running problem and I noticed the oil filter had a drip on it. There was also a small wet spot on the gravel underneath. I thought it might have been from coolant that leaked on my first attempt at changing the thermostat. When I smelled it it smelled like gas. The dipstick did too and it hadn’t before.

I think my many attempts to start it when I wasn’t getting spark and to read the EEC codes with a jumper wire severely flooded the engine. My ignition switch on the steering column is worn so sometimes finding the sweet slot where it would display codes would cause the pump to prime the engine multiple times. While I guess it could be caused by a lot of things my thought was that it was so flooded it moved past the rings and into the oil pan.

I’m gonna drain it in the morning and replace with fresh oil and get it towed and cross my fingers it’s not a gonner. Hopefully it’s not too expensive a lesson to learn. I’ve really enjoyed working on it and hope I didn’t fix my self out of a classic.

 

So I got the truck towed to a mechanic who was recommended by a friend for working on old fords. Very nice but very brief. Called me back and said it was an ignition switch. I asked if he could also install the temp sender I had purchased and left in the cab once I decided the truck had enough of my “help”. Picked the truck up, starts up every time and runs smooth. Idle air control valve is hooked up and truck idles well. Temp gauge on dash works too.

As the mechanic was pretty brief I wanted to look over what I had done to see if any of the coolant hoses or the new thermostat housing I installed leaked. I noticed he hadn’t topped off the radiator so I did that and then took for a quick drive with heat on. Got nice heat, everything under the hood looked dry except one small spot. The new thermostat housing I installed has a threaded socket screw or plug right above the port for the upper radiator hose. When new it had thread sealer around it on the interior. The old housing did not have a threaded opening there. Around the top of the socket screw or plug there was a green ring where the coolant leaks just enough to pool around the plug. It also looks like it maybe threaded in there at an angle. I wondered if the mechanic had used this to drain coolant to install the temp sender and possibly cross threaded when it was installed. I didn’t touch it when I installed the new housing.

My question is should I drain system, remove plug, add some thread sealer (or rtv?)? Or if it isn’t leaking too much should I just leave it? If it is cross threaded I’m guessing I’ll need to get a new housing and replace as well?

Any help appreciated, thanks!

 

No. Don’t just leave it. The system is pressurized to increase the boiling point. Hopefully it isn’t cross threaded. Use either tape or liquid thread sealer: not thread locker or RTV

 

Thanks My4Fordtrucks for your help. That make sense about the pressurized system. I will replace and hope it’s not cross threaded.

 

Read some of the forums on here about oil choice for older Fords. I used 5w-30 motor craft synthetic with a f1a filter but noticed the oil pressure gauge runs within normal but on the lower side.

Was curious if I should switch to 10w-30 or use a conventional (non synthetic) 5w-30?

Also curious as I continue to have questions with my truck if I should just keep to this thread, or start a general one with my trucks specs as the title. It’s been running great since the issues I’m aware of have been fixed but am past the ignition issues but didn’t want to load up a new thread for every question. Thanks!

 

That oil pressure gauge looks like a gauge, but, sadly, actually functions like a dummy light. It just shows oil pressure or no oil pressure. Different people prefer different oil weights, but I like to run 10w-30.

 

I use Motorcraft oil since it is priced cheaper than a lot of other oils. It is semi-synthetic. The recommended weight is 10W-30 unless it is going to get below freezing then 5W-30 is suggested.

 

I fixed the leaking socket plug on the thermostat housing with thread sealer and refilled the coolant. I had to top off the reserve tank once but levels seem to be holding steady.
Trucks been running very well.

Been driving to and from work, about 20 minutes, and the temp gauge slowly climbs to a bit past midway, to the A in NORMAL, and then backs off just past midway right in the middle of the M. I’m guessing this is the thermostat opening. It then largely stays steady right there and occasionally moves a little higher or a little lower.

Are these slight fluctuations normal?

I also noticed after installing the new thermostat that the new one is a 195 degrees while the one I took out was 192. Is this why it’s steady point is just past dead center? Is that 3 degree difference enough to be worth switching it back out to a 192? I couldn’t find a motorcraft one at first but now I have.

Probably paying too much attention but it’s all new to me.