Exhaust Smoke going Downhill

[Stock90IDI]

I'm having a curious problem with exhaust smoke from my truck that I hope someone here can shed some light on.

I’ve noticed my last couple of road trips that under certain conditions it produces a lot of exhaust smoke. It only happens when driving downhill with the throttle slightly depressed, just enough to maintain a constant speed. I’m not towing and no heavy loads in the bed and speed doesn’t seem to matter much as it happens anywhere between 40 to 65 mph, as long as I’m maintaining a constant downhill speed. With my foot off the throttle or when accelerating, it stops smoking. Also, it doesn’t smoke at all driving around town.

I'm the original owner, truck is bone stock, all original with 53k miles. Engine starts easy, idles smooth and accelerates normally, none of which produces any smoke. Mileage is decent at close to 16mpg highway and all else seems to be operating normally.

Any ideas on what could be causing it?

 

[Thewespaul]

Do you happen to have issues with your fuel gauges? Sounds like you may be sucking air when at that specific angle. Probably an issue with your fuel pickup (shower head)

 

[79jasper]

That's a good thought.
I was thinking timing. (Actual base timing or the advance lever on the pump)

Sent from my SM-G892A using Tapatalk

 

[Stock90IDI]

Thank you for the quick response.

The rear tank does have a broken shower head and gauge wanders around so I only go 150 miles before switching tanks or refill. Front tank pickup and sending unit/gauge currently seems to still work fine - I routinely go 240-250 miles until refill.

However this most recent trip I only used the front tank and the truck smoked on a downhill shortly after a refill so don't think it was sucking air due the angle. Maybe if shower head was cracked and about to fall off but I would think it would then also suck air traveling level at lower fuel levels.

 

[Macrobb]

It's either air or timing at this point.

 

[Thewespaul]

I would think it would do it more than in that specific situation going downhill, and with having gauge issues already my money is on air.
Although most IDIs I have come need their timing to be adjusted.

 

[pelky350]

Mines used to smoke a lot before I change out my injection pump downhill/decerleratiNg. If you are sitting still and rev to somewhere over 2000 rpms and hold steady will it "cough" whitish smoke out the exhaust pipe? It could be timing retarded but pump could Be bad also. In my case It was both, advancing the timing helped but did not solve, new pump solved it

 

[Stock90IDI]

Mines used to smoke a lot before I change out my injection pump downhill/decerleratiNg. If you are sitting still and rev to somewhere over 2000 rpms and hold steady will it "cough" whitish smoke out the exhaust pipe?

Well I just read your post and ran right out to try it out. Held it at 2200 rpm and she belched white smoke and ran rough. She definitely does not do that running 2200 rpm on the highway though.

Guess I'll check the timing to start with. I don't have the correct tools but found some good advice on the forum about a good digital timing light and Ferret tool so will be ordering those straight away.

The IP is the original one with 53k miles but it is 27 and a half years old. Not sure how long they should normally last. Are there any other tests I can do to verify it's the problem or contributing to it?

 

[Thewespaul]

Try filling the fuel filter with atf and running the engine for while to fill the ip with atf and leave it like that overnight

 

[Stock90IDI]

Try filling the fuel filter with atf and running the engine for while to fill the ip with atf and leave it like that overnight

I assume any old ATF will do? I have some Mercon type ATF left over from changing the transfer case fluid a few weeks ago.

 

[Thewespaul]

No, clean unused atf.

 

[Stock90IDI]

No, clean unused atf.

Sorry Paul, my bad. I meant the unused clean ATF leftover as I bought a 1/2 gal and didn't need it all. I'll use it.

I was reading through some older posts about this procedure and some members recommended running it hard (like you stole it) until the ATF is flushed through the system. Is that the norm?

 

[pelky350]

Cleaning may help it, my truck was a sitter as well I just got a good deal on a lightly used rebuilt pump at the time and just swapped out. But when I was tying to forgure out my problem was told pump is likley going out by international mechanic. My truck didn't smoke while highway driving either. Only smoked while decelerating or free reving and holding. I would try advancing the timing some first and the atf trick cleaning pump and see what it does for you before replacing pump.

 

[Macrobb]

The reason it smokes decelerating or free-revving is because there is no load on the engine. No load = very little fuel injected, which means that the plungers have to squish further before the fuel starts being injected. This results in the beginning of the injection cycle being later.
The IP compensates for this with various timing advance mechanisms, which end up being sticky or wearing out after many miles.

More than likely, the internal advance piston isn't doing it's job correctly right now, so you are getting across-the-board retarded timing. You only get to the smoke point, though, under "no load" high RPM conditions.

If the ATF doesn't fix it, you can always just advance the timing a bit. It'll probably take quite a bit of advance to do much, though.

 

[Stock90IDI]

Cleaning may help it, my truck was a sitter as well I just got a good deal on a lightly used rebuilt pump at the time and just swapped out. But when I was tying to forgure out my problem was told pump is likley going out by international mechanic. My truck didn't smoke while highway driving either. Only smoked while decelerating or free reving and holding. I would try advancing the timing some first and the atf trick cleaning pump and see what it does for you before replacing pump.

Sure sounds like mine has the same issue with the pump. The timing light and Ferret Injection Detector are ordered and on their way. I'm currently up in Oregon for the holidays but will advance the timing and flush the IP with ATF when I get back to Cali.....definitely expecting it to smoke on the downhill sections going back. One side benefit though is it sure keeps the tailgaters backed off behind me when it smokes.

The reason it smokes decelerating or free-revving is because there is no load on the engine. No load = very little fuel injected, which means that the plungers have to squish further before the fuel starts being injected. This results in the beginning of the injection cycle being later.
The IP compensates for this with various timing advance mechanisms, which end up being sticky or wearing out after many miles.

More than likely, the internal advance piston isn't doing it's job correctly right now, so you are getting across-the-board retarded timing. You only get to the smoke point, though, under "no load" high RPM conditions.

If the ATF doesn't fix it, you can always just advance the timing a bit. It'll probably take quite a bit of advance to do much, though.

I very much appreciate the detailed description of the how and why of the IP workings and how it's likely causing the smoking issue. I'll definitely report back on the results after doing the IP flush and timing adjustment.

I must say that although I joined the forum 6 years ago and this is my first time posting to the forum, I have found the members to be a wealth of knowledge on the workings of these trucks and very helpful is sharing that knowledge to help other owners of these trucks. I've learned about repairing window motors, cavitation, coolant types and coolant filters, as well as broken shower heads just to name a few things. This forum is such a great resource and it's clear that the members here have a true love of these trucks. I hope that I can contribute and share something in the future to help another member in need.