Calling All Tbi Experts...

mjlimo82 · February 1, 2011

If there's one thing I don't know much about on these trucks, it's the fuel system (I can work on Holley TBI's on a Dodge or any carburetor LOL). I've got a strong fuel smell that approaches every once in a while that is accompanied by sometimes a low idle and a "flooding" scenario (although not always at the same time). I'm thinking pressure regulator in my mind, but I need someone to confirm or throw some ideas at me before I start replacing stuff that doesn't need it. Also, when going down the highway at 55 or so and you let off of it to slow down for traffic/lights/etc, when you go to step back on the gas the engine has to catch back up with the speed of the transmission. Acts like it's losing fuel pressure to me. I've also noticed it has a rougher idle that it did a month ago - where it once ran extremely smooth, now it has a shake almost like it has a cam in it. Fuel pump is only a year old and it's a Bosch so I'm hoping that's not the problem because I don't want to pull the bed off again (who does?). Since I've had it, it sometimes takes two tries to get it to start when it's really cold out - it'll fire on the first try but immediately dies; turn it over again and it fires and stays running. When it's not cold out it fires, stalls, then picks up and runs. 1990 Sierra 350/4L60, all factory original minus a K&N and true dual exhaust. Ideas? Thanks guys!!

ChevyTech77 · February 1, 2011

Check the fuel pressure. I use AC-Delco fuel pumps exclusively for a reason. I also find it easier to replace those pumps from underneith. Dropping the tank is no big deal.

Never seen a fuel pressure regulator fail on a TBI truck before. Check for injectors leaking down after you shut it off.

The true dual exhaust will cause problems with the EGR system on these trucks which can cause weird driveability issues. On the older applications, I just delete it but I'm not in a emission regulated state, either.

GMstrong · February 1, 2011

Yea my 90 sierra is being a pain in the ass toooo.cant get her to idle at all.she runs fine with the map sensor hooked up.The fpr on tbi is easy i actually just took mine all apart yesterday.How about your fuel filter?How does the injector spray look?

GMstrong · February 1, 2011

without the map sensor hooked up i meant.

mjlimo82 · February 1, 2011

We're not emission-regulated either, and I've thought about getting rid of all that anyway to be honest. Guess I should go ahead and look into it. The knock sensor needs replaced too and I'd just do away with it at the same time. Easier than changing it out in my opinion.

What's the ideal pressure at the injectors supposed to be? IIRC, it seems like 12-15 is what I've seen others say is normal.

Filter is fine, and the spray looks pretty good to me. Looks like a normal pattern as it should. I have another TB I've considered swapping over just to see what happens with it, but I've yet to give it a try. Thanks for the help guys!

mjlimo82 · February 1, 2011

PS I've noticed my mileage has gone down also. Changed the filter at that time but no change - it wasn't all that dirty anyway. Before I swapped motors this past summer it was run on 93 octane otherwise the old motor wouldn't run. Had a dead cylinder and 87 just wouldn't cut it. Backfired, bucked, etc. It wasn't pretty.

ChevyTech77 · February 1, 2011

9-13 psi of fuel pressure is spec.

Make damn sure all of the engine to frame grounds are good and connected along with the main battery ground to the engine block. If the engine to frame ground staps look like they are green, replace them. Make sure the grounds are spread between the two thermostat housing bolts too. These ecm's were temperamental with ground issues.

mjlimo82 · February 1, 2011

I'm almost certain that they aren't spread across the t-stat housing if I remember correctly, so that's an easy fix. The main battery ground is good, but don't know about the engine to frame straps. Makes sense because that's another issue I've noticed at night with the lights on - they flicker when the truck is cold, and the volt gauge jumps up and down. Only when cold though... once it warms up it's fine. One thing leads to another...

Thanks for the help!

ChevyTech77 · February 1, 2011

Make sure the positive cable on the starter is tight too then. Seen that before also.

mjlimo82 · February 1, 2011

That was the first thing I checked when I noticed the flickering, and it's fine. I had to put a new starter on it over the summer and double checked to make sure it didn't work itself loose. All this started when I replaced the motor so I'm sure there's something I left unhooked somewhere, but I sure as hell can't find it!! I'm going to for sure fix the t-stat housing ground wires and also look at the ground straps. 20 years old so I'm sure it wouldn't hurt to replace them anyway.

mjlimo82 · April 15, 2011

Well so this has only gotten worse... I swapped the TB last night and now it's idling too high and when you stomp on it, it falls flat on its face. Can't tell if it's flooding or choking, but there's definitely something amiss. I swapped out the TB (which had different sensors on it) as well as the EGR sensor and nothing changed for the better. I hate asking dumb questions but I don't know enough about GM TBIs to do any good with it on my own. It runs so bad I can't even drive it. I put a new gasket on it when I swapped TB's by the way... checked all that was suggested previously in the thread also. I also removed the air filter to make sure it wasn't choking it out (even though it's pretty clean) and it made no change in the way it acted, and I can't find any vacuum leaks with carb cleaner's assistance. I have no clue what to do at this point, but I sure miss my truck... can somebody help???

92rado · April 15, 2011

Well so this has only gotten worse... I swapped the TB last night and now it's idling too high and when you stomp on it, it falls flat on its face. Can't tell if it's flooding or choking, but there's definitely something amiss. I swapped out the TB (which had different sensors on it) as well as the EGR sensor and nothing changed for the better. I hate asking dumb questions but I don't know enough about GM TBIs to do any good with it on my own. It runs so bad I can't even drive it. I put a new gasket on it when I swapped TB's by the way... checked all that was suggested previously in the thread also. I also removed the air filter to make sure it wasn't choking it out (even though it's pretty clean) and it made no change in the way it acted, and I can't find any vacuum leaks with carb cleaner's assistance. I have no clue what to do at this point, but I sure miss my truck... can somebody help???

Have you tried the temp sensor? Sometimes that will cause them to run really rich.

Squirrely · April 17, 2011

I wonder if your temp sensor is toast, or going bad. This can cause odd idle issues.

If you stomp on the gas and the truck falls flat then you're pulling too much air. Fuel pressure may be off. I'd turn it up if you can.

The only other thing I can think of that may save your sanity is to take the truck in, pay 80 bucks to have someone in a garage tell you what the hell is wrong. At least then you can take it home and fix yourself without dumping a ton of time and money into it.

mjlimo82 · April 19, 2011

It was the coolant temp sensor, so thank you to EVERYONE that helped! The truck runs better now than it has since I swapped motors last summer; no more "miss" when it's idling in park or in gear. I can't thank y'all enough... I know very little about GM TBI systems so all the input y'all provided is MUCH appreciated. I never would have imagined that SOB would cause it to run so bad.

Squirrely if I had the money to take it to the shop I would have. Anything I can resolve myself with the help of others is that much easier on my wallet (but not always my sanity). My good friend is a mechanic, but working on mostly newer stuff and having only been in the field for 7 years, he's not seen too many GM TBI's come in the shop; they're mostly Vortecs so this was Greek to both of us. Either way we learned a lesson and will keep it handy the next time it quits. I'm sure that won't be too long down the road since I had to buy a BWD part. The store that carries Delco parts closes at 5 PM.

92rado · April 19, 2011

Hats

It was the coolant temp sensor, so thank you to EVERYONE that helped! The truck runs better now than it has since I swapped motors last summer; no more "miss" when it's idling in park or in gear. I can't thank y'all enough... I know very little about GM TBI systems so all the input y'all provided is MUCH appreciated. I never would have imagined that SOB would cause it to run so bad.

Squirrely if I had the money to take it to the shop I would have. Anything I can resolve myself with the help of others is that much easier on my wallet (but not always my sanity). My good friend is a mechanic, but working on mostly newer stuff and having only been in the field for 7 years, he's not seen too many GM TBI's come in the shop; they're mostly Vortecs so this was Greek to both of us. Either way we learned a lesson and will keep it handy the next time it quits. I'm sure that won't be too long down the road since I had to buy a BWD part. The store that carries Delco parts closes at 5 PM.

That's good I'm glad that worked for you! Its always the little things that will drive you crazy!