'94 350 TBI Start and Idle issue

[Viru]

Hey all,

I've started having a strange issue since I cleaned up my engine bay a couple of weeks ago. I used a spray instead of power washing and blocked off as many sensitive areas as I could when cleaning.

The truck now has trouble starting, but will do it after a lot of cranking and some starter fluid. Once started, it idles fast and revs higher before shifting gears. The fast idle also causes it to shift into gear very aggressively.

There are 2 unidentified cables that have always been disconnected in the engine bay, and the plug for the idle adjustment screw has not been removed.

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https://photos.app.goo.gl/95gHu43JhJctLq8V7

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I've never had an issue like this before, and would really appreciate the help!

 

[Viru]

An update,

I managed to fix the idle by cleaning and readjusting the IAC valve, and the rough shifting seems to have been because of low fluid.

However, still dealing with higher shift points (sometimes won't shift before redline), and tough starting (looks like the oil pressure takes time to build up). It shifted perfectly fine when I drove it to work, but after work it went back to high shift points. Starting seems to be an issue only when cold.

This is getting stranger by the minute and I can't figure out why.

 

[Gramps]

Check your fuel pressure. I suspect it’s low. Could luck out and just be a clogged fuel filter. Seen that many times.

 

[Viru]

I'll follow that lead! What's the best way to test the pressure, in your opinion?

 

[Urambo Tauro]

That's a great question that probably deserves its own thread, weighing the pros and cons of each type. Since these TBI trucks don't have a schrader valve, you have to undo one or more connections and temporarily install an gauge in-line.

There's an adapter that's about 4-1/2 inches long, which goes in place of the fuel filter. There are also some other types that will let you leave the filter on, but you still have to undo a connection and maneuver it into place.

A little bit of googling suggests that the TBI inlet, as well as both ends of the filter are all M16 x 1.5 thread, so it looks like you have some options as to where you'd prefer to tap in at. This is handy if you want to compare pressures before and after the filter (or maybe you'd rather just change the filter, if it's overdue for replacement).

Haven't had to mess with it yet myself, but if the time comes that I ever need to test my fuel pressure, it looks like I'll be stuck with only being able to tap in up at the TBI inlet, because I modified my filter setup to use quick-connects.

 

[Viru]

Thanks for the info! I'll have to look around for a gauge I could use, otherwise might just replace the fuel filter. It's only $5 on RockAuto and I got the truck recently so have no clue how old the current one is. Just hoping the fuel pump is fine because that's not a job I want to do in sub-freezing temperatures.

That's a really interesting mod! How often do you change your fuel filter for that to make a difference?

 

[Urambo Tauro]

Well, I did that shortly after buying the truck a couple of years ago, so I've actually never had to mess around with the stock one. The fuel and brake lines were junk, so while I was making new ones I decided to do that mod. Based on past experiences with GM products, I knew keeping the old threaded filter setup was going to make future maintenance a hassle, so now that I'm back to driving a GM of my own again, I decided to nip it in the bud before it had a chance to become a problem. The filter's not due to be replaced for another year or two, but I expect it to be a piece of cake.

 

[Viru]

Makes sense. The thing that's still bothering me, though, is that the truck was starting/running/driving absolutely perfectly until I cleaned the engine bay - these issues started almost immediately after. I'm still baffled as to whether it's a coincidence or I screwed something up.

 

[Viru]

Update, it turned out to be a few things.

- Replaced my IAC because the cleaned one jammed, must've been on its last legs, helped the idle further.
- Replaced Fuel Filter and reset codes, which solved the starting issue, but was still shifting hard and giving me a CEL.
- Replaced TPS, which got rid of the CEL, perfected the idle, and fixed my shifting issues!

As it would seem, I somehow fried my TPS when cleaning the engine bay, and this just made all of the other issues far more apparent. Strangely, it didn't give me this code until after the Fuel Filter and reset job.

Either way, glad to have the vehicle running better than ever!

 

[smdk2500]

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As it would seem, I somehow fried my TPS when cleaning the engine bay,

I did the same thing. Degreased the engine and power washed it off. Shorted the tps and Ignition module. Not sure how i did the module but I did