Narrowing down common TBI problems...

[Fireball5657]

Hey all, it seems like everyone with a TBI motor is trying to fix bad idle, lack of power, and poor fuel economy, I'm no different. Common issues, but the only problem is that there is rarely ever just one magical cure-all to these issues, people think they have the answer because it worked for them, but someone else will replace that same part (say the TPS or something) on their truck and it won't make a difference. Watching posts on the internet over the last two years has taught me that as much as people like to rag on the idea of throwing parts at a car, that's really the only solution for engines like these.

My truck is a 94 with the TBI 350, rebuilt 4L60E, 4x4, extended cab, 3.73 gears and a G80 and just over 268K miles without a major rebuild, a prime candidate for the everlasting TBI issues. In the past I did plugs, wires, cap and rotor, I've replaced the fuel filter, and one of the injectors were replaced ~12 years ago. Other than that, it's all stock!

Checking the SES lights shows that I had a code for a MAP sensor and the EGR, both of which could be causing these issues. So I started there, I scraped together what leftover money I had after bills tonight and bought a new MAP and IAC (SES said an EGR, but I didn't have the money and it'll get replaced anyways.) and if replacing those two parts made a difference, it was a minimal one. Idle improved slightly, it no longer stumbles when in gear and at a stop, but it's still not as smooth as it should be. Power improved slightly as well, it lights up the rear tires at a dead stop now when the pedal meets the metal, but after takeoff and when rowing through gears, power didn't improve much. Downshifts are torquier though!

Despite my truck being an automatic, it has a tach and one thing I've noticed about the motor is that it runs out of steam at about 3800 RPM, it'll usually go to 4500 RPM before it shifts, but it just seems like the motor is spinning, but its being choked by something. I know that the exhaust and the heads are major bottlenecks on these engines, but I've driven a low mileage 95 that was equipped just like mine and it had power until about 4100 RPM, and even when it shifted at 4500 it still had more power left then mine despite them both being completely stock motors. I'm guessing my issue is more with the intake rather than the exhaust. A few thoughts I've had is that the air filter is clogged and isn't allowing the engine to breathe enough, or maybe the fuel pressure regulator isn't getting enough fuel to the engine (not sure on that one though.) Whatever it is, it's choking up the engine, and that could be the root of my problems. (lid flip just makes it louder, no power difference for me)

a bit off topic, but my 0-60 times are around the 13s range, what are everyone else's? Is 13s normal for a 200hp 4x4? I got smoked by a friends stock 99 Ram with the 5.9, 4 auto, and 4x4, and he has a 3.42ish axle ratio!

Next paycheck will most likely be vacuum lines and other rubber stuff, nothing is visibly bad, hoses still are solid, but the gasket on the stock IAC fell apart when I took it out so I know the rubber isn't in great shape. EGR will also be next since it's a known issue.

As far as other suspects go, I'd say the fuel pressure regulator and injectors, the O2 sensor, or maybe the air filter as mentioned earlier.

Anyways, anyone have any thoughts or wanna add anything to this?

 

[man-a-fre]

I'm following this one one ,have the same problem with my new Mexican gm crate 350 ,in my 94. Replaced plugs, wires, Iac ,cap rotor ,oxygen sensor ,tps, cts,fuel pressure regulator set at 13lbs ,fuel pump ,new cat ,air cleaner flapper door working properly, set timing to 0 wire unplugged timing jumps to 20 or better wire plugged in, injector pattern looks good , but i'm going to send them to Mr injector Bill Johnson in Idaho to flow and clean the factory injectors in hopes it will gain me some power. I even did the tbi smooth over mod removed ridge and such and spaced the injector pod up and added an airraid tbi spacer with no real improvement . I am hoping the flow matched clean injectors are what is needed to fix the power issue. Don't know if the computers get lazy with age and slow to adjust but maybe that is our problem may need a new ecu?

 

[Schurkey]

Stop replacing parts, and start TESTING parts.

What is your fuel pressure under heavy load?

What is your exhaust back-pressure?

What is the cranking compression of each cylinder?

What is the scan tool telling you about misfire counts per cylinder? O2 sensor cross-counts? Other sensors/outputs?

There's thirty more tests you should run, and a dozen tune-up items to inspect.

 

[man-a-fre]

What scan tool gives you misfire counts per cylinder on a 94 ? I have and use a mt2500 and an actron cp9690 and can't seem to find misfire per cylinder data in the menu on it.

 

[someotherguy]

Hey all, it seems like everyone with a TBI motor is trying to fix bad idle, lack of power, and poor fuel economy, I'm no different. Common issues, but the only problem is that there is rarely ever just one magical cure-all to these issues, people think they have the answer because it worked for them, but someone else will replace that same part (say the TPS or something) on their truck and it won't make a difference. Watching posts on the internet over the last two years has taught me that as much as people like to rag on the idea of throwing parts at a car, that's really the only solution for engines like these.

The last sentence of your first paragraph is directly contradicted by everything preceding it. The logical explanation for why throwing parts at problems is not universally successful is because guesswork is not troubleshooting. There is not one magical cure-all for most problems on most vehicles; you don't simply throw parts at them no matter what brand is on the grille.

By the way, your friend's '99 Ram 360 has a good 50hp on you so it's not a good comparison.

Stop replacing parts, and start TESTING parts.

What is your fuel pressure under heavy load?

What is your exhaust back-pressure?

What is the cranking compression of each cylinder?

What is the scan tool telling you about misfire counts per cylinder? O2 sensor cross-counts? Other sensors/outputs?

There's thirty more tests you should run, and a dozen tune-up items to inspect.

This, this, this, and more of this ^^^ as far as a scan tool that will tell you misfires per cylinder on OBD1, dunno, but instead of targeting what may not be readily available, all of the other tests are meaningful and important.

Spend some money on some basic troubleshooting tools instead of just more parts. The parts guesswork may or may not yield positive results; the tools will serve you many times over.

Richard

 

[Schurkey]

What scan tool gives you misfire counts per cylinder on a 94 ? I have a mt2500 and an actron cp9690 and can't seem to find misfire per cylinder data in the menu on it.

Ooops. My mistake. TBI system doesn't provide that info. Sensor data is available, though.

 

[PlayingWithTBI]

EGR will also be next since it's a known issue.

It may not be a bad EGR valve, the passage through the intake manifold from the head to the EGR mount may be plugged up with carbon - known issue too.

As stated above, get the proper diagnostic tools instead of throwing parts at it.

Don't know if the computers get lazy with age and slow to adjust but maybe that is our problem may need a new ecu?

No, the PCM doesn't get slow with age like most of us do. The earlier ECMs ran at 160 baud rate where the later ones like the 7427 (for 94 - 95) ran at 8192. That's why a lot of folks like to upgrade from the 7747 to the 7427 along with the ability to control the "E" transmissions.

 

[thegawd]

I know it happened to me and I almost never see people talk about it when troubleshooting bad idle and poor performance.... due to a leaky vacume brake booster. on trucks this old I bet a lot of them have a leaky brake booster that the owner has gotten used to.... as in used to the sound and the terrible performance as it gradually fails.... I'm not sure how to test this with an obd1 but on an obd2 you press the brakes and watch the short term or long term fuel trims rise and when you let off and press the gas it falls on its face.

Al

 

[1997]

I know it happened to me and I almost never see people talk about it when troubleshooting bad idle and poor performance.... due to a leaky vacume brake booster. on trucks this old I bet a lot of them have a leaky brake booster that the owner has gotten used to.... as in used to the sound and the terrible performance as it gradually fails.... I'm not sure how to test this with an obd1 but on an obd2 you press the brakes and watch the short term or long term fuel trims rise and when you let off and press the gas it falls on its face.

Al

Old school vacuum gauge works

 

[PNW_GMC]

I've got a 91 suburban v1500 with 98k miles. It was running like a dog, but got hard starting and stumbled til it got warm. We did the parts cannon thing. IAC, EGR, tune up. . .. I'm sure it was the coolant sensor that did the trick. It has not run this good in 20 years. I think all the parts I replaced contributed to the end result, not just the coolant temp sensor. After 30 years it doesn't hurt to change all those things out. All together it wasn't $400 and worth every penny if you want a great running dependable ride.