Hello! I've been searching the forums for almost a year now and I've finally hit a level of frustration worthy of posting. First a little back story...

Last summer I inherited my dad's 85 F150 2WD 4.9L with 130k on it. He'd had it since 2010 when he bought it off the original owner. It's about as bone stock as I've seen, minus one big issue. Dad had issues with it running way too rich that his mechanic couldn't fix after multiple trips to the shop (he had no experience with the feedback carb system). Finally he recommended that my dad buy a non-feedback carb off an older model and have it installed (but didn't recommend the other steps of the DS2 swap :/ ). He did, but he never did any of the rest of the DS2 swap for the ignition. The only remaining vacuum lines connected are the MAP sensor, the EGR, the PCV, and the main booster to the brakes. After that, he swore it ran great and gas mileage improved. Fast forward a bit....

I used the truck sparingly until it became my daily driver 2 months ago. Now that I'm running it daily, I notice a lot more about how it runs and drives and it's really struggling. It starts right up and idles pretty well, and it runs under load up hill just fine, but maintaining speed and light acceleration under little to no load results in terrible sputtering and what I think is misfires. I read a bunch of posts about this that recommended changing the fuel filter, plugs, and wires, so I did. I haven't replaced the distributor cap yet but I have carefully cleaned the contacts.

Finally last night I decided to disconnect the spark plug wires one at a time to check for misfires and cylinders 1,2,5, and 6 cause the engine to bog down badly as expected, but cylinders 3 and 4 cause no change. I swapped the wires around to rule them out and no matter what, its cylinders 3 and 4 with the issues.

So that's where I'm at. I've read swapping the carb without changing the distributor, module, and wiring harness can cause a limp home mode but I can't find any documentation on that. I've read the codes with a ODB1 scanner and it only throws the idle air control valve and throttle position sensor codes which makes sense seeing as they dont exist with the non feedback carb.

My question is, would the lack of complete DS2 swap cause cylinders 3 and 4 to behave like this? I had planned to complete the DS2 swap this summer (I'm having trouble finding the wiring harness or a guide on patching the existing harness to the DS2 hardware) but I'm hesitant to put any more money in it if this cylinder issue is more likely to be a head gasket or something else catastrophic. I have next to nothing invested, so there's that, but I need a reliable daily driver for work and I'd really like it to be this truck if possible and financially responsible.

Thanks in advance for any advice and for reading my long winded question.

 

Welcome aboard! A compression or leak-down test will show if there is a mechanical problem. I purchased a 1986 with 4.9L and feedback system [in limp home mode] from the original owner, who had let things run down. The first and second tanks returned a dismal 9 and 11 mpg and the engine would buck badly on deceleration. I am going to attach a link to a report I made on this. Maybe it will give you food for thought.

https://www.ford-trucks.com/forums/1...-9l-300-a.html

 

Thanks for the info and link, that post gives me some hope. I have a friend who offered his compression tool to test that as soon as I can get over to his place to borrow I plan to do that. The symptoms you describe in that post are nearly identical to mine so I'm guessing its the carb / ignition mismatch causing the limp home mode and that's hopefully the main problem.

When you did the duraspark conversion, did you find a wiring harness on a junkyard or did you splice the factory one to use the duraspark components? I've seen a few places that splicing is possible and even found an email addressed in a very old post offering a copy of some home made plans describing how to do it but when I sent an email the gentleman's wife responded saying he had passed in the years since that post :/

 

As said a compression test is the next step.
When you have readings on all holes post back what they are so we can guide you in the next step to take.

If it is not over heating I would say it's not a head gasket but with 130k a valve job is most likely needed.
Dave - - - -

 

So 2 quick questions in the interim until I can test compression...will running in the limp home mode damage the engine in the short term? I know the truck needs floor pans and a little bit of other body patching to pass inspection and I’m hesitant to invest in the rest of the DS2 swap if I can’t get it inspected for another year so I’m debating putting that on hold until I get a fresh sticker

2) if compression is good, could being in limp home mode be my only problem and competing the ds2 swap be all that is necessary?




thanks!

 

The only thing I know being limp home mode dose is hurt performance and MPG other than that I can't think of anything else.

I can't see doing the DSII swap to help with them 2 holes not doing much help when the plug wires were removed.
IIRC you replaced the plugs & wires maybe the new plugs are bad? You can try and put them 2 plugs in other holes that worked to see if they do anything.
If you have a vacuum leak at the manifold to head for them 2 holes thatwould also make them not fire.
But the true test is a compression test.
Dave - - - -

 

Ok so I tested compression. - 120psi across the board.

i also checked timing and it was showing ~18 degrees advanced. I set back to 0 and it ran like crap, so I adjusted back towards 10 and it’s running ok.

Cylinders 3 and 4 are still not firing based on the spark plug wire disconnect test. At the recommendation of a mechanic friend I sprayed some starting fluid on each intake port of the manifold and 1,2,5, and 6 showed no change but 3 and 4 both caused a rev in RPMs and the engine smoothed out to better than I’ve ever heard for a few seconds. This leads me to suspect the manifold gasket has failed around 3 and 4 but I still wonder if the carb/dizzy/module mismatch isn’t my only issue.

Truck is going to the shop Wednesday to see what all it’s going to require to get a sticker and (fingers crossed) assuming the list is manageable I’m planning a trip
to a u pull it lot to hunt the rest of the DS2 swap parts soon.

 

Try this: https://www.ford-trucks.com/forums/1...expensive.html

 

Ah that seemed like such a simple fix! Unfortunately every port on my tree is sealed tight other than the ones still in use.

 

Perhaps, but if smoke comes out where the gasket is for the 2 cylinders in question, you will have confirmed a vacuum leak.

 

Ah didn’t think of that. Looks like I need to go get a cigar. Will report back later on, thanks!

 

See with the compression now known to be good and the spray, I like carb cleaner, at the intake & head you know where to look closer.
I can tell you the gasket is about $10 it will be the R&R to do the job that will add up if a shop does it.
It is not a hard job if you want to do it yourself and being you said something about hitting the P&P for the DSII parts you might want to do the job.

BTW when at the P&P maybe look for a set of EFI exh. manifolds and Y pipe. Do a search for EFI manifold swap it is not a hard swap and being you have the manifolds off for the leak what the heck!
The hardest part of the swap is needing 3 hands to hold the manifolds to get the washers & nut on.
Dave ----

 

Cigar test made a liar out of me...there was a cap on the vacuum tree that failed. Smoke pouring out of there, the junction between the EGR and the base it mounts to (that gasket must have failed) and from below the 3rd cylinder intake port between it and the exhaust port. So I guess that confirms the manifold gasket needs replaced. But fixing the failed vacuum cap smoothed the idle out a bit, and I’m guessing a new EGR gasket would help too.

one other DS2 swap question...assuming I find the parts and do the swap, where should the timing be set for that? TDC (0
degrees) or something else?

 

Start @ 10 degrees BTDC.

 

DS2 swap is deceptively easy. Takes like 2 hours. You can buy the pig tails with connectors online new for about $12. You will have to make the harness yourself if you can't find any ford product 1976-1984 ish (84-88, Ford- 1991 Jeeps if you count very heavy duty vehicles. Since you have a square ignition coil, buy a DY-210 red grommet module.

I think you can re-use your plugs, wires. cap and rotor. You also need a distributor. Buy a 1980 model to be safe. All you need is dizzy, module, and pig tails. Maybe some misc. screws and wire