1993 K2500 454 Falls flat, pops back, Low and high idle RPM.

[WarWagon]

Working on a buddy's 1993 K2500. Started just running bad one day while being a DD.

Just put a new fuel filter on and no change. Distributor was replaced in the past decade so any OEM issues with it would be less probable.

Stomp it from a light and it throws you back a second then looses power and pops back.
No power at all over 3000 RPM. You literally have to back out of it so it can shift up.
Idle was high over 1200 RPM warmed up at a stoplight.
Idle RPM is low after startup hot.
Has to spin over several times to start hot or cold. Used to be turnkey hot.

Watching the idle air motor positions change on the scanner, but, not sure if it's actually moving. (Will pull it at some point and clean it out.) Idle all over the place doesn't act like it. Was over 45 MPH for 15 min so it should have learned the IAC position by now.

Scanner indicates very few O2 cross counts. Hangs either rich or lean. Getting on the throttle has it go extremely lean and stay there. Even floored. Crosscounts look good at a light throttle steady speed. But only then.

Timing light failed to work after a few flashes. Untested light so maybe a dud. Can't find my battle tested one at the moment. Was going to check FI patterns with it.

Going to pull the cap and check the rotor next. Just in case it's reading lean on the O2 sensor from a bad ignition.

That or EGR flopping open is all we can come up with at the moment.

Maybe we got more than one problem.

Ordered a new O2 sensor to have one on hand... The parts counter asking What bank and pre or post cat was hilarious. The deer in the headlights look when I said it only has one... No movement till I reminded em it's a 1993! On reflection as it was reading decent crosscounts on the way back from the parts store so IMO the one in it is fine.

 

[MrMarty51]

If it were Mine, I would begin with an entire tuneup. Plugs, wires, cap and rotor then see where that leads.
Also good idea on installing a new O2 sensor, but do one, or the other first so if the tune is the fix then You will know positive that was the problem, or, the O2 sensor first, but road test between each one.

 

[MrMarty51]

High resistance in plugs or plug wires will cause the running conditions You are describing.
Remove the distributor cap, check and see how much carbon is atop of the rotor, and, how much carbon buildup inside of the distributor cap.
Especially around the spring loaded center post of the cap.
Any amount of carbon indicates bad plug wires or bad plugs.

 

[Will L.]

How much money can be thrown in to do tune up is a question to follow up w/ Marty’s idea. But idk if I would install all that just yet- bad new parts theory.

I’m sure you:
pulled the air filter and inspected for rats nest and its New fuel.

Most likely air/fuel ratio issue.

First hook up a vacuum gauge to the intake manifold.




Next is dry and wet compression test. I know - jumping into the deep end quickly. But knowing for sure one of the three legs of this three legged stool is correct is worth it. Then at this point I jump in with Marty and spin in new plugs at least.

When pulling the plug wires you’ll have a feel for if they are 400 years old. A dark nighttime look under the hood for plug wires sparking around is good if you have the chance and money is tight. The poorboy method I had to do more than once is engine running and starting at top of plug wire loosely grip the wire and slide my hand all the way to the plug. Failed wires will give you the shocking reality you need to replace them. No shock doesn’t mean they are optimal or not broken inside, but moving the wires while you handle them, if it causes engine stutter does that part. So dont be easy, shake it around as you go. Do this before pulling the plug wires.

You can also do the quicky test of lay the plug again engine one at a time and crank to watch each one have good spark at the plug. This is a requirement if not replacing the plugs when out. Remember to clean and gap and always check gap on new plugs- seems a common misnomer is new plugs come gapped-nope.

Check that actual timing when checking your fuel spray pattern when ya get that good one found.

 

[ak diesel driver]

From your description it sounds like it's running lean. Maybe a failing brake booster?

 

[WarWagon]

That's a good idea I can read the MAP off the scanner for the intake vacuum.

No sign of rats nest in the intake.

Oddly it has two PCV valves on the engine. Looks like a screw up as the "fresh air" from the air cleaner housing has one shoved on it at the driver side valve cover. IMO should only have one PCV from the passenger valve cover to the TBI unit.

O2 sensor was replaced first when this problem showed up. Than parked for a year or so. Put new gas in it last night, in addition to the 10 gal tossed in it a month ago to start it up, as it was parked empty and had the tank siphoned for another vehicle to get places... No change.

Has a MSD coil and plugs wires are only a decade old. Not a lot of miles on them. But I have been surprised at failed plugs and wires before. @Will L. going lean will light up brand new wires as the spark wants to go somewhere. My 1985 Olds with a failing MAF would light up 8.8MM plug wires when it went lean. Aka Lean condition blew a bunch of holes in the plug wires.

Went through quite a bit to get this running when he got it like 12 years ago as a non-runner. Never solved the warming up idle hunting. Heated O2 mod was suggested way back then, but never done. Went through the throttle body, new dist, O2 sensor, fuel pump (one in tank was not that old he found then.)

I would think a vac leak would "improve" the further the throttle is opened as it becomes less of a air source esp at WOT.

 

[WarWagon]

From your description it sounds like it's running lean. Maybe a failing brake booster?

He even replaced the brake booster in the past. Don't mean it doesn't have to be tested though.

PCV oddity:

 

[Will L.]

No, you have to have air inlet to the crankcase somewhere. The pvc is sucking out air. This means fresh air has to come in somewhere. Usually it is through the air filter housing via a hose to the opposite valve cover. iirc inside the filter housing is a small 1” X 2” tiny filter where the hose connects. Then that hose goes to the driver side cover. Been a lot of years though.

Yes bad booster becomes a horrible vacuum leak. Hence the vacuum gauge test. Takes. Like 30 seconds and tells of any vacuum leak along with other problems.

As to lean fuel causing plug wires to fail- never heard of that. Think that was coincidence. Ignition system should fire year after year regardless if it is putting on a light show being used as training tool in classroom (seen that more than once) or actually lighting fuel on fire. The amperage load doesn’t change based of fuel quality or quantity.

 

[WarWagon]

As to lean fuel causing plug wires to fail- never heard of that.

After you blow a hole in an insulator ... Like going lean so it's an easier path to go through the spark plug cable vs. fire the plug. But it's now a hole in the spark plug cable. IMO It's not as good of an insulator after those events that make lots of visible sparks.

Yes it can still fire the plug on the proper mixture afterwards. My favorite is a bad coil to dist wire failing under load and leaving telltale marks on the coil wire. Regardless that HEI system on the 3.8, a one year only thing that was replaced by 3 coils in 1986, lit the spark plug cables up bigtime in the dark when the MAF went lean. It would stall the engine just by tapping on the bad MAF and/or smoke black flood it out. I warrantied out the 8.8mm plug wires after it was finally fixed. I am surprised it didn't blow a hole to the coil through the cap like the HEI's loved to do.

 

[WarWagon]

Tossed a timing light on to check injector patterns and watched my buddy jump back like "INCOMING!" when it shot flames 4' out of the TBI popping back. I didn't even flinch and asked: "What? This has been popping back: what did you expect? BTW... Do I still have eyebrows?"

New plugs and wires. YaY! ACDelco sold itself out to Communist China for spark plugs. Made in China on the box now. Guess NGK Japan wasn't cheap enough for them anymore.

Regardless bone white insulators reading Lean Lean Lean. All 8. Plug wires all broke leaving the connectors on the spark plugs. No other problems seen with the ignition.

It likes more air during the "Choke" time after startup. I experimented with the brake booster line and full air and choking that line off. Runs better after the "Choke" part of warm up is over.

Looked at the TBI and wondered if a gasket was seeping fuel. Keyed it on a couple times and ...

Fuel pump sounded like it was crushing gravel. So new fuel pump planned to be installed.

Found out damper has "slipped". Timing mark nowhere to be seen with timing light. Damper on order for tomorrow.

 

[ak diesel driver]

I thought this rig was gone?

 

[WarWagon]

I thought this rig was gone?

Didn't sell because it runs so badly.

 

[Will L.]

Gonna find out it is super advanced once the new balancer is on.
Don’t forget the new seal - those never survive a balancer swap.

 

[WarWagon]

Found the problem. Cracked fuel hose on the fuel pump assembly in the tank. (Not an AC Delco Part.) Runs like it should now. Further no more idle speed hunting. We followed the advice to use a 1996 Vortec pump that isn't maxed out by the stock power level on the 1993 454.

Reference:

TROUBLESHOOTING | TBI

 

[MrMarty51]

You use those same plastic hose clamps ?
Just curious, I have reused those many times.
Brand new Dodge van, would not start. I was new to the TBI systems. Removed the air filter and cranked over the engine. I could see it was squirting fuel but was it enough.
Took a pump style squirt oil can, dumped the oil, filled it with gasoline, cranked over the engine and give the throat of the TBI a couple of squirts of fuel. Engine fired right up.
Dropped the completely full fuel tank, pulled the pump. One of them plastic type clamps had not been clamped securely over the tube area that the hose slipped over, pressure pushed the hose off and it was not delivering full pressure.
Repositioned the clamp, tightened it around the hose and all was fine for the life of that van.

 

[WarWagon]

You use those same plastic hose clamps ?

It comes as an assembly with the hose. We are not replacing just the pump, but, the entire module with level sensor, sock etc. The old hose was also 'loose' so yeah don't think much of the clamps. They only got to last longer than the in-tank pump/motor lifetime warranty, err um, vehicle gets gone for some reason or warranty forgotten.

 

[SnowDrift]

Found the problem. Cracked fuel hose on the fuel pump assembly in the tank. (Not an AC Delco Part.) Runs like it should now. Further no more idle speed hunting. We followed the advice to use a 1996 Vortec pump that isn't maxed out by the stock power level on the 1993 454.

What caused you to suspect the problem was inside the tank? Fuel pressure check?

 

[WarWagon]

(Not an AC Delco Part.

Well maybe it was (or a knock-off).

What caused you to suspect the problem was inside the tank? Fuel pressure check?

Watching O2 sensor cross counts ... Only light throttle would there be any. Heavier throttle it went "lean" and stayed there. Spark plugs read lean. So a fuel problem.

Timing light check of the injector spray pattern showed noting unusual.

Listening to the pump during key on: it didn't sound good. As above unusual noise.

No easy way to test fuel pressure on this year TBI and no tool to do so.

So we tossed a part at it. (The engine damper is slipping so timing is also suspect.)

 

[SnowDrift]

If it's original, then it gave up the ghost after 30 years? - not too bad.

 

[Will L.]

Fuel pump is definitely not an original.
Those pumps die fast from plugged fuel filters. Factory recommendation for the fuel filter was every 10,000 miles- so every 3rd oil change is common practice if 3,000 mile interval.

We did oil changes at 5,000 miles rather than 3,000 in the big fleet & we buy fuel filters in case quantities of coarse saves money. So after hiring a young kid year, 2 years in -accounting noticed we had been going through fuel filters way to much. We noticed the fuel pumps had stopped dying and thought ac delco fixed their goof up. No- turns out the kid misunderstood and was replacing the fuel filter every oil change.

When the account came down to talk about it, us shop guys were having lunch and one of the GM engineers was with us that day working on the 6.5 waterpump testing project. The engineer, without looking up or missing a beat,He described-The gas fuel pump was engineered stronger originally and got redesigned so that it would cost them a couple dollars less per unit- which is mega millions in savings by production numbers- but fail not much out of warranty period. He said there was debate to tell customers that doing the fuel filter every 6,000 miles would greatly extend the life of the pump. But they figured customers would just get mad and demand a larger fuel filter to last longer.

The account ran the numbers on filter cost vs pump cost (which was crazy expensive back then, prices fell after 2005), down time, etc. and then we started changing fuel filters in the gas trucks every oil change.

For those unaware the gas fuel filter is exactly in place where the diesel lift pump is. If you ever wonder why the lift pump is shaped & sized like it is- mass production of fuel lines and assembly line speed are the answer.

For people wondering why the 6.2/6.5 pickups never had in tank lift pumps- they tried. They couldn’t survive- the thicker fuel being hard to push killed the pumps just like a moderately restricted gas filter does.