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Stalling / backfire right after startup


Darel

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'86 BII 5sp

Having an annoying issue with my BII. When you go and start it cold, it will start and idle fine. High idle is OK, normal idle is fine too. However if you try to drive the truck anywhere, it just won't. It will gag and sputter and misfire, and backfire too under a load. Clutch in / no load and it idles and revs fine. Only when it's got a load on it.

However, this is only for the first 5 minutes or so. Once you manage to stumble a quarter mile or so, it'll be A-OK and run fine all day long, around town and highway driving.

So far, I've replaced the IAC and the coolant temp sensor (for the ECU). When I first got the truck two years ago, I also gave it a full tuneup and replaced the distributor with a reman unit, keeping the original Ford module because the generic module that came with the reman dizzy was bad.

I will also note, sometimes I get some gray/black smoke from the exhaust. Like, at first start, and when it's sitting idling for a long time - for instance, when I was charging up the A/C and it basically sat in my driveway idling for about two hours.

Any thoughts?

Thanks,
Darel
 


Darel

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Replaced fuel pump last month. New filter when I "recommissioned" the truck two years ago, no crud in tank when I replaced the pump. Also, it'd be worse when warm / floored and it's not. I'm not ruling out some sort of injector issue (more likely an injector control issue) but there's nothing that tells me the fuel system is bad.
 

Darel

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I should note, I am no stranger to brand new parts being bad out of the box, however the truck has done this since we've had it, and no changes I've made have had any effect at all. Could it be a timing issue? The backfire leads me to that.
 

littlefoot

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yeah if its the timing it could even just be an issue with the timing belt? unless you've changed that already as well?
I should note, I am no stranger to brand new parts being bad out of the box, however the truck has done this since we've had it, and no changes I've made have had any effect at all. Could it be a timing issue? The backfire leads me to that.
 

Darel

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These have a timing chain.
 

JerryC

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Thinking out loud...
First five minutes bad, then good - open loop issue then 02 sensor warms up and goes into closed loop and can correct fuel issue. So where is the problem? bad temp sender (replaced but not new at this point) or TS wiring, vacuum leak, TPS issue (could maybe be carbon buildup in throttle body keeping the blade from fully closing, ACT sensor???

Is the smoke part of the problem? Black smoke=fuel, grey=oil. If fuel, I'd look first at dirty injectors..
If grey, then I'd guess valve seals. Oil seeps in as the vehicle sits and will also build up in valve cover as the vehicle idles (probably lots oil gunk on the head slowing the return flow oil (my 86 had this issue).
 

Darel

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Hmmm.....vac leak. That's a possibility. What in the vacuum circuit is open in that first few minutes but then closes off?
 

JerryC

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Hmmm.....vac leak. That's a possibility. What in the vacuum circuit is open in that first few minutes but then closes off?
It's not that the vac leak goes away, but that the 02 sensor sees it and calls for more fuel to fix the lean condition. When I got my current BII a few years ago it had several small vacuum leaks through dry rotted hoses and bushings. I found those and then later the upper intake gasket cracked from old age (it made almost no power when this happened).
 

Darel

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Gotcha. Believe it or not I actually did smoke test this a month or so ago when it was acting up and didn't find anything, but I also didn't really put a lot of effort into it. I'll have to dig into that a little more. Thanks!
 

JerryC

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Check the ACT, Air Charge Temp sensor on the passenger side of the intake. It may be bad or just covered in goo.
 

Darel

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For $8 I just ordered a new one. Not even worth my time taking the old one out without replacing it. Will update. Thank you!
 

Darel

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By way of an update: I checked the timing, it's perfect. (plug out, checking off passenger's side front plug wire, right?).

Replaced the intake air sensor. It was in terrible shape, completely coated in soot, and the plug itself had some corrosion inside and looked like it wasn't making good contact. Didn't really change much though.

Now, the problem seems to have changed and I am leaning towards the ignition module. I can start it and it will idle fine. It will move under its' own power. It won't get far though, before it just completely loses power. Won't stall, but I can floor it and it will barely respond. Then occasionally, VROOM! the truck seems to realize the accelerator is floored and shoot forward. Then right back to bogging again. It can barely drive on a level road.

I have ordered a new module (the original is still on there) and TPS. I am leaning towards the module because when it's idling, it will seem to rev and respond in a normal, linear manner. It just won't do anything with a load on it. Also, it doesn't seem to be getting better when the truck warms up now, either.
 

JerryC

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Is there black smoke pouring of the exhaust when this happens? This doesn't sound like a spark issue to me. Weak or no spark you have lots of unburned fuel and stalling.
IMHO, it sounds like low fueling or huge vacuum leak. The way you describe how it drives sounds just like when mine had the upper intake gasket split. It idled fine but felt like it had a 10HP.
 

Darel

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No smoke. Guess I'll start tracking the vac leak. Thanks!
 

JerryC

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If it is a vacuum leak it shouldn't be hard to find.. Have you put a vacuum gauge on it?
 

Darel

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Another update.

I apologize but I haven't been able to dig too deep into this because I just don't have the time. I have to work through things in tiny 15-minute bite-sized chunks.

Anyway, over the weekend I smoke-tested the entire vacuum system. No leaks. I pulled every plug and checked every line and the manifold. Nothing.

I replaced the TPS and ignition module. I had them anyway. I still have my cold starting / stalling / running rough problem, however, I was able to drive the truck normally once it was up to full operating temp. I drove around the neighborhood for about 20 minutes and it was fine. I had to move along to other things (remember bite-sized) so I parked it to cool down and I am going to go home and see how it does from a cold start.

I put a vac gauge on it. Steady ~17 at idle. Seems normal when I blip the throttle. I thought that was pretty good for a 300,000 mile engine.

Fuel pressure is a very steady 32 psi at the rail, I know 40ish is what you're looking for but I don't put a ton of faith in my old tire pressure gauge held on the valve. All I know is it seems to be enough pressure and it's steady regardless of how I rev the throttle.

My plans for today are:

See what happens from a cold start. I am not optimistic.

Pull all the vac points off the manifold and plug them. If it runs better, start adding them back one by one to try and find the leak. I don't think there's a leak and this will probably waste a lot of my time.

I think the module was the source of the no power / surge / undriveable problem. When I took the old one off the bolts were really, really loose. It may have started to back out of the plug. Or it could just be bad.

How do these ECUs deal with low / dead batteries? I know more modern computer-controlled stuff needs to see a minimum of 9.4v or thereabouts. My kid keeps killing this battery and I think maybe it might be on its' way out. Starting the truck isn't easy, and I thought it was just dirty terminals so I cleaned them and charged the battery, but starting is still like...RRRRT...............................RRRRRT..............................RRRRRRR. Very slow as if the timing were too far advanced (which it's not, I checked). Is it possible the computer really needs to see more voltage until the alternator kicks in, in order to properly control the system?
 

Darel

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Another update.

Truck sat on the charger all night. Fired it up and immediately backed out of the driveway and started driving around. No issues. I do think it's still stumbly and needs to "clear its' throat" when idling, which is pretty much what started all this and needs to get fixed. But it was totally drivable the whole time, even before it warmed up.

Pulled in the driveway, shut it off, immediately tried to restart and the battery was dead. So that's a problem. I'm still showing 14.2 at idle though so it's just the battery. Given that I have 14.2 I don't think I can blame low voltage for the rough running.

Also tried pulling the vac line to the cruise control and no change in driving or vac readings. I tried that one because it was the only one I wasn't 100% confident on after the smoke test (with the line that runs into the firewall and all). Didn't bother with any others because it ran fine.

Will have to get an new battery and repeat this trying to flush out the crappy cold idle. I am confident that module was an issue, but that it was only one of the issues.
 

JerryC

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Vacuum, 17 seems low but I think mine was about the same the last time I checked. I was hoping for more like 20, but I wrote that off to cheap gauge. Low generally means weak rings but can be affected by leaks and timing. Was the gauge steady, or did the needle flicker?

Off the top I don't remember which is which but there are two colors of TFI's, black and gray. One of them is good for the BII and the other is not.

Battery, swinging wild azz reasoning here: the battery is part of the electrical system and who knows what it is doing unless you scope it out. It could be shorting internally causing electrical noise and messing with the comp and sensors. The alt could be driving so hard to recharge the battery it could be sending who knows what electrical signal trash.

Grounds, while we are thinking about possible electrical issues how are the chassis grounds? There should be battery to frame, frame to engine, engine (driver side head, rear) to firewall and battery to core support.
 

Darel

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Honestly I know vac on a good engine should be 20-22 but I've NEVER had an engine read that high. I'm good with 17, especially at 300k.

There was an ocasional "stumble" on the needle, not rhythmic, and not sharp like a sticking valve or anything. Honestly the vac gauge tells me nothing that I'm actually going to act on (this truck will never see a re-ring, or head work, etc....)

Good point on the grounds. Maybe I'll whack those with the wire wheel. Just fixed one on the wiper control box yesterday. Also good point on the elec system....probably best to hold off further troubleshooting until I get that battery.

Thank you!
 

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