Hi again all. I have a new problem. The truck is an 85 5.0L w EFI.

I went and got my truck aligned yesterday, and when it was ready the guys in the shop said they couldn't get it started and that they had to push it out into the lot -- which surprised me, because I've never had trouble starting it except once.

Anyway, so I went out to where they pushed it and sure enough it didn't start. I kept cranking and eventually it started to fire up, but died. A few more tries and I was able to give it enough gas to stay running. The truck was stumbling pretty badly, and when I put it in drive it pretty much died on me. So I put it back in P and let it run for a while which didn't improve how it was running. So I shut it off, and then started it up again, and immediately it was running better somehow -- motor wasn't stumbling. I still let it sit for a few minutes, and then drove away -- and everything was fine for a while.

I jumped on the highway for a couple of exists and everything was fine, got off and continued home. After about 10 or 15 minutes of driving I noticed it was running crappy again. The truck would run ok if I kept the gas light, but would stumble to the point of almost dying if I gave it too much gas, and I noticed it was running rough when sitting at stop lights -- so basically it was running the same as it had been in the parking lot.

So I kept driving it home and periodically tried to accelerate to see if it would choke on me. The werid thing is that for a while after it was running badly it started to run ok again ... i would floor it, the engine would ramp up, it would downshift and off i would go. But again after a while it ran s@(#* again. I would floor it, and the thing would buck badly and almost stall.

So eventually I got it home and started searching. The problem seems VERY similar to this thread here, but no one really figured that one out.

If it was a fuel delivery problem, would you expect it to buck when accelerating or just not accelerate? I didn't think it would buck. Unfortunately, there's no schrader valve on the fuel rail, so it's a little tricky for me to check fuel. The truck is relatively new to me, but one of the tanks is new and the filters look new. Air delivery is simple, so I'm not really leaning in that direction. Electrical seems most likely to me -- because the thing seems to be running ok and then not and then ok again, but I'm at a bit of a loss for where to start looking. Any ideas? Is there a computer that handles igition timing in this thing or is it just the distributor? There's a big cable that comes off the distributor and (i think) goes to a module on the side of the engine compartment. What does that thing do?

I appreciate your help!

 

From what you describe could be many things -- vacuum leak, EGR, ignition module, or fuel system...etc
I would do a diagnostic test / code read and see what you come up with.
See links in my signature below for how to read codes and also some fuel system info.

 

klricks, thanks for the suggestion I will check the codes and see if they shed some light on this.

 

Check your fuel filter and check your fuel pressure. Fuel pressure should be around 35-45 psi IIRC. Sounds like maybe your fuel pump is starting to give but like posted above check them codes!

 

Mine was acting very similar and it ended up being the fuel pump relay on the drivers side firewall.

 

OK... so I finally had time to get back to this tonight.

First, I checked the codes.
I had one code: 63 TP voltage below threshold.

So I started it. The truck fired right up and died immediately. I tried again and the same thing. On try 3 (I believe) the truck stayed running. I went to the front and worked the throttle, and the truck choked. I let the truck idle and it died.

I disconnected the TPS and started the truck again. I think the truck ran better (but it's a judgement call). I could reproduce the choke when throttle was open affect.

I shut the truck off and reconnected the TPS and started it up again. This time when started it was running much better. Idle was higher, and the truck was running smoother. This time, no amount of messing with the throttle made the truck choke. I let it run for about 15 minutes while I was doing other stuff, and periodically opened the throttle with no problems.

Is that it then? It was just a bad throttle position sensor connection? I suppose it would explain the werid behavior if the connection was just marginal. What do you think?

 

I've encountered this kind of behavior before, that unplug/replug trick worked for me for a short while (maybe a few months) before the problems reappeared.

Rock Auto has/had certain Motorcraft factory OEM TPS thingers on sale for cheap due to a wholesaler's closeout, might check their site.

 

OK I started the truck up this morning and it was the same problem. I looked in the chilton manual and I couldn't find anything about testing the sensor. The sensor has 3 wires and it appears its some arrangement like power, signal (out), and ground. I took a meter and the power has 5 volts on it and the signal wire varies from 1v when throttle is closed to 5v when the throttle is wide open. Does this sound right to you?

 

I have never tested a TPS or the wiring so I don't know what "normal" is. I'd look here for a general guideline:

Fuel Injection Technical Library

Ya can't rely only on the measurements at the two extremes... these things are just rheostats; have you ever fiddled with the volume **** of an old radio that makes a scratchy noise as you turn it? Same thing.

Haven't looked at the Chilton's for a while but I seem to recall it having some decent no-start diagnostics, not sure if I saw TPS stuff there or not.

 

Thank you very much for posting that link. I looked at it very quickly, but it seems extremely useful. It does appear that the TPS is actually behaving more or less correctly though. That's interesting, I'll review the web page in more detail and test some more, and then I'll go through the chilton manual carefully and try to test all the ignition components. If I can't find a problem I'll move on to fuel delivery.

 

My truck would idle just fine, then die when I romped on it. That was the fuel presure regulator.

I also had a no start issue lately. It ended up being the magnetic trigger on the distributor as well as the ignition module.

Ive had poor idle issues as well, but I think it was caused by my idle set screw being jacked with before. I unplugged my IAC and started the truck. It ran...which is bad. The truck shouldn't run with the IAC unplugged. I adjusted the idle set screw to where the truck would just die with the IAC unplugged. I plugged the IAC back in and the truck is doing much better now.

 

Well I am confident it's not the fuel pressure regulator because I changed that recently.

I really didn't have enough time to check the ignition fully tonight because I got home rather late, but I did put a spark tester on the secondary of the ignition coil and got a pretty healthy looking spark. I will check the spark at the individual plugs (or test at least a couple of them), but if it all checks out may lead me to fuel testing.

Unfortunately, I don't have a schrader valve on the fuel rail. Does anyone have a simple suggestion of how I can test the fuel pressure without a valve on the rail?

 

Have you tried a different TPS?

 

No I have not, but when I look at the thing with a meter it appears to work the way your link describes. Should I replace it even though it seems to be ok?

 

I don't know... if the computer complained about it once already, in my experience, the computer is having problems with it and it will be complaining again; the unplugging/replugging thing didn't work real long for me.

The problem with a JY TPS is, you don't know if it's a known-good device or not. But it's a different device, and you could swap them and see how the truck reacts (can swap it back afterwards if you want).

But I probably wouldn't replace it just yet; if it were me, I would probably first test the fuel pressure. You don't have a Schrader valve, are you sure? I thought all these EFI things did.

But I wouldn't be entirely surprised if you don't have one, my DD Tempo with TBI doesn't have one, either, even though some of the books claim that I do.

I don't have one of those EFI fuel pressure testing kits, either. My Tempo has TBI (throttle-body injection) which uses a relatively low fuel pressure, ~15 psi and this is within the capabilities of a "normal" vacuum/pressure gauge. I merely disconnected the underhood fuel filter and installed the gauge after the filter but before the throttle body. There are some electrical connections on the computer diagnostic plug that can be jumped to make the fuel pump run full-time.

My fuel pressure was less than half what it should have been, POS Spectra Premium product I had at the time.