My brother has a 1988 Ford F-250 he just replaced everything but the block (351 Windsor Carb). When it runs it runs great. However it seems to make it just over 50 miles before it quits. He has tried replacing the bent rod and shorter plugs. The issue isn’t that they are getting gummed up the most recent change in spark plugs turned out to be burnt. He has gone through all of the possibilities he can think of (and he is a trained mechanic) if anyone has any ideas as to what the issue could be please chime in.
The first thing I would check for is a faulty ignition module.
The ignition module functions in two modes. These are the start and run modes. When starting the engine, the ignition module goes into the start mode. The ignition module allows full battery voltage to the ignition system. This ensures there’s a hot enough spark to start a cold engine while at the same time drawing current from the battery to start the engine. Once the engine starts, and the ignition switch moves to the run position, the ignition module switches to the run mode. The module then steps the voltage down to the ignition system because now the charging system has come on line. So it basically works the same as when ignition systems had ballast resistors/wires.
So it could be that the ignition module is failing to go to the run mode once the engine starts, so now the coil is getting too much voltage burning up the plugs.
Tester
Please describe “burnt plugs”…The FIRST thing that shows damage with an overheated spark plug will be the ground electrode…the sharp edges and corners will become smooth and rounded…So will the center electrode…Like Tester, I would be looking at the ignition module and possibly the coil…Next time it quits, feel the coil. Is it really warm, like hot? If so, I would replace both parts…
This is a TFI ignition model and it’s quite easy to advance the timing way too much. This can burn up plugs at a minimum and ruin an engine at a maximum.
Has the module been changed, ignition timing checked or altered, etc?
The SPOUT connector MUST be disconected before checking and setting the timing.
Just pointing it out in case any of that is applicable.
The complaint of burnt or gummed up plugs is a little hazy though.
I think OK’s on to something…
I was also curious about what you meant when you said he ‘tried replacing the bent rod’
Oil pressure switch is also something to be aware of. That vehichle DOES NOT use an inertia switch for the electric (yes,it is electric) pump–it uses an oil pressure switch located at the rear / top of the engine. It is bypassed in crank position. For all it matters, yiou can bypass it with apaper clip in order to test it when you have trouble…The ignition modules almost never fail on these…this is not TFI it is DSII.
Why would it not be a TFI? It’s an '88 model with a 351W and as far as I know these were TFI vehicles. Websites for several parts houses show a distributor and TFI module as the proper fit.
There are not many 1981-1989 vehicles on the road today for good reason…
I’ve seen a weak spark cause plugs to darken prematurely with soot.
In the opening, the it is said to be a 351W Carb,… There were E/F250s up to '88 greater than 8500GVW that were Holley 4V Carburated. TFI was never coupled with a carburated engine. Only Duraspark was used. No other electronic controls are present. Duraspark module or asswociated component failures are pretty rare.
“Duraspark modules or associated component failures are pretty rare.”
I don’t think so. The Fords I’ve owned that had the Duraspark ignition systems all had a second ignition module mounted to the fender well. Why? These modules were notorious for failure. So when it did, all you had to do was open the hood and plug into the second ignition module and you were on your way.
Tester
Y’all (Okie vernacular) are probably more familiar with the old Ford minutae than I am.
I vaguely remember my manuals saying Duraspark was dead by '84 with Lincoln using TFI that year followed by the rest of the Fords changing over starting the following year.
Being a bit curious now, I checked 3 websites for various parts houses for an F250 with the 5.8 Windsor and they all showed it as an FI model with a TFI module being offered.
There was no mention of a carburetor option. ((I thought TBI had taken over by 83 or 84 with MFI grabbing hold by 86. Memory is fuzzy on this.)
The OP was a bit short on info anyway with this “bent rod and shorter plugs” bit and the comment about replacing everything except the block. What does that mean? Changed parts right and left to make it run or does it mean a modified engine?
If for the sake of discussion this vehicle does have an incarnation of Duraspark on it and the DS module is not the problem then maybe there’s a fault with the Hall generator in the distributor. I’ve seen a number of these that were faulty and yet performing the test on the distributor with a Snap-On tester showed they were fine.
The same could be said for both the DS and the TFI module. They can test fine for eternity and still be bad.
Run the engine and feel the coil. If it’s hot, it’s getting too much voltage.
Tester
Tester:
I’m confused when you say too much voltage at the plug is causing the plugs to burn out. The voltage at the plug will only rise to whatever is needed for a spark to jump the gap. Once the spark is initiated, the voltage no longer continues to rise.
There was no clean switchover from one system to the other but if and only if, the engine is carbureted, then it’s ignition system is guaranteed to be DSII. There weas never a throttle body injected version in the US and only a few elsewhere and they were EECIII which never had TFI. 1992 was the year that carburetors disappeared from all but one model in the US Ford lineup. The Crown Vic police version offered the 351 VV carb up to and including 1991. The 351 and 460 truck engines were still available with carburetors as late as 89. However, the later 351 and 460s were in non-traditonal stripped chassis such as F57(you won’t see that listed in any parts book) used for things such as FEDEx local delivery vehicles and the like. The 429 went to port fuel injection in '92 andthe 370 was discontinued in '92. Just remember though that TFI never appeared on a carbureted engine. Also, Ford had but one and only one feedback caburetor and that was the VV carb on the Crown Vic police version. I was quite involved with that era and stand by the durability statement of the Duraspark modules. The only failures I experienced were corroded wring and misdiagnosed problems.
Thanks for clearing that up about the carburetor fit. I’m familiar with TFI not appearing on a carbed engine. I just wasn’t aware Ford used carbs into that era.
I also don’t have a problem with DS, consider it a good system, and superior reliability wise to the TFI. The DS problems I’ve seen were comparatively few (how many problems out of a gazillion of them being manufactured) and generally involved a distributor fault rather than the module itself.
The TFI gets a few choice words though… 
It isn’t a TFI it is a carb. The bent rod thing i recalled him saying something but to put it simply he replaced the entire distributor. he did rebuild the entire engine with a bit of modification. if you were to imagine taking the entire engine apart and you are left in the end with the block and you didn’t put any of the original parts back on thats what i meant by he replaced everything but the block.
and he says the timing is perfect