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TBI experts! I need some help!

S

Scrubby

1/2 ton status
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Boise, ID
The truck is an 1989 K30 with the 454 TBI. It seems to run pretty well, but recently the gas mileage has gone from about 8-9 to around 4 mpg. It also has the following problems or symptoms:
  • At cruising speed on the highway it seems to cough or lose power causing a bucking sometimes.
  • It doesn't have the power going up hills that I would think a big block truck should have. It does not have enough power to stay in the top gear of the TH400 on a decent highway grade. It will slow down until it shifts. I would think the stock 454 should be able to pull a grade at speed with 4:11 gears.
  • I have seen the check engine light on once or twice for a minute or so, then it when out.
  • From time to time, I am getting a sulfer smell out of the cat.
  • When starting warm sometimes it will die and be hard to start. Seems like a vapor lock, but the elec. fuel pump shouldn't allow that right?
Anyway, I have replaced fuel filters and replaced the O2 sensor. Some one suggested TPS next or the MAP. I have also heard the cat could be clogged. It was taken to the chevy garage 6 months ago (when the gas mileage was still good, but the power still sucked) and they said the power problem was the transmission. I think that was BS. Anyway, I am guessing the computer is not throwing codes, or the check engine light would be on full time? The plugs do not look fouled. The actually look like the A/F ratio is pretty right on.

What do you guys think the next step should be?
 
Check your fuel pressure, yeah the smell from the exhaust might be something, but the symptons you are describing sounds just like when my pump was losing pressure.
 
Sounds similar to the problem I had with my TBI 350. Turns out it was a fuel pressure problem. I narrowed it down to the fuel pressure regulator. I bought a kit with gaskets and a diaphram to rebuild the regulator. When I took it apart to rebuild it I was amazed at how filthy it was inside, covered with sixteen years of gunk. Anyway, after that and a fuel filter it ran fine. I suggest that that be on the top of your list of things to check out. Also, the ECM will store any trouble codes that it throws, even if the light goes out before you get to it. The codes are only erased if you disconnect the battery for a few minutes. You can check the codes with a paperclip. I think the code for a lean condition (fuel pressure problem) is 43.
 
Another theory could be bad injectors.

At cruising speed on the highway it seems to cough or lose power causing a bucking sometimes.
Click to expand...

Not injecting enough (or too much) fuel into the engine

It doesn't have the power going up hills that I would think a big block truck should have. It does not have enough power to stay in the top gear of the TH400 on a decent highway grade. It will slow down until it shifts. I would think the stock 454 should be able to pull a grade at speed with 4:11 gears.
Click to expand...

Same thing "Not injecting enough (or too much) fuel into the engine"

I have seen the check engine light on once or twice for a minute or so, then it when out.
Click to expand...

Not injecting enough fuel (lean condition) "check engine light" comes on

From time to time, I am getting a sulfer smell out of the cat
Click to expand...

Pumping too much fuel (unburned fuel gets into cat causing smell)

When starting warm sometimes it will die and be hard to start. Seems like a vapor lock, but the elec. fuel pump shouldn't allow that right?
Click to expand...

Injectors pumping too much (or net enough fuel)

If your injectors are worn out, then they loose their ability to meter fuel properly for any given condition. Incorrect fuel pressure can be a contributing cause as well, which is maintained by the fuel pumping system. I am thinkng fuel system problem.
 
1) Check codes...could be a VSS issue...
2) Replaced O2 so that rules that out...
3) Check the basics...plugs..ohm the wires..cap...rotor..timing...fuel psi...etc.
4) Cut that cat outa there...

Obviously with the sulfer smell thats probably going to be the issue...cats seem to last 8 to 10 years before they plug up...just my experience. But if you fail to make all the basic checks you could miss something...

EGR malfunction would show up at idle since the ecm controls the vacum valve that regulates when and how much exhaust gas is recycled into the combustion chamber...and either a sticky pintle of vacum diaphram failure would result in the motor chokeing itself at idle...

Doug Wright
 
Thanks guys. Sounds like I should check the fuel pressure. That would explain the vapor locking. If you have anymore ideas, let me know.
 
all of the above...and...

I had a guy I know who'd bought an 89 C3500 Crew Cab truck with a 454 TBI brand new complain to me he felt it never ran good after the first 6 months he had it..it was getting 12 mpg when he first bought it,and he said it felt very peppy ...then it gradually started getting "doggy",and on cold days in the winter it ran even worse,less power, and mileage dropped to between 6 and 8 mpg..he assumed it was just the colder weather ,he knew ALL vehicles use more fuel when its cold out..

He had a tune up done at the dealership..new plugs,wires,filters,cap and rotor..truck only had 20,000 miles on it,so they did it under warranty,and no codes were stored in the computer(check engine light never came on once)..they also checked the catalitic converter,said its was ok..

(I've seen clogged cats do the same to other trucks,and sometimres they only "clog" once in a while,when broken chunks of the insides get lodged in the pipe outlet area)..

My brother looked the truck over at his shop,and he noticed the heater was not putting out "hot" heat,like his truck did..the owner agreed the heater was not the greatest,but he didn't think much about it..then my brother put a thermometer in the radiator..the coolant was 165 degrees,after the engine being driven 10 miles to his shop,and left idling for at least 20 minutes!..

He went to the GM dealer,and bought a new AC/Delco thermostst,a 195 degree one as specified in the trucks owners manual..and installed it..they go for a ride...

The guy returns with a big grin.."My heater works better than ever,and I can FEEL the difference in power already!."..a few days later,he returned to tell us his truck used only ONE tank of fuel instead of the usual two going to a job in NH,and it seemed to be running better each day..

We autopsied the old thermostst..it was sticking open slightly,and looked like it was never closing completely..like one that had been damaged from overheating..evidently it never let the motor warm up enough to get the computer out of the "cold" range,and run off all of the sensors....the owner did say on warmer days it sometimes ran good,and got decent mileage,but noticed as soon as it got colder out,the truck really started running doggy,and wasting gas..

He took it back to the dealer,and told them what we'd done to "fix" it..they looked at it again,and found it had a faulty EGR solenoid too,that caused it to buck once in a while when he towed his trailer..it never set off the "check engine" light though!...it wont hurt to check your EGR valve system out,and your coolant temparature,and the fuel pressure..:crazy:
 
I forgot to mention. I ran the vacuum pump on the EGR test and it killed the engine when I put a vacuum on it, so I am assuming that is ok.
 
Check the codes before you do anything else. CEL does not have to be on to set trouble codes. Trust your ECM computer. It knows a lot more than you do. My best guess is you are Prolly getting a 42. If so either your pickup coil or dist module is the problem
 
Is there a good reference out there for how to check the codes? Someone said you can do it will a paper clip. How do you do that?
 
Just because it throws a code, doesn't mean 100% that this is the problem component,,,when my fuel pump was runnin weak, it kept throwin the code for the oxygen sensor, so i replaced it, and then it would throw a code for the idle air control valve, then the water temp sensor, but mostly the o2 sensor.

fuel pressure is not that hard to check, from my personal experience with this motor, thats the first thing i would check with the sympton you are describing.

As far as the injector, i had one crap out on me as well, it would just shut down the motor for a split second or two, then fire right back up, until it progressivly got worse, then just wouldn't start.

looked at it one day, when i turned the key on, it literally dumped fuel into the intake, filled up the "bowl" of the intake.

$150 later at Napa, new injector:( :D
 
Ok, I checked the codes and there were none. It just kept flashing 12. Next up will be checking the fuel pressure. Here is the kicker though, it has 2 tanks and it runs about the same on both. That seems to me it is not fuel pressure since both pumps are doing the same thing. I'm starting to think plugged cat. Any way to make sure of that other than just replacing it? I'm also going to check the thermostat.
 
You can measure exhaust backpressure by drilling a small hole in the exhaust pipe, ahead of the cat. Attach a pressure gauge (0-10 or 20 PSI) and see what the backpressure is at 2500 RPM. It should be pretty close to zero. If you see much more than 1 PSI then you've found the problem...
 
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