Weird Chevy truck problem

[tracto-man]

Maybe a little OT, but trucks are tool and we use tools to work on them right?

A friend and myself got to messing around with this 1998 Chevy K1500 5.7 350 4x4 pickup. It runs okay but if floored or right about 2500 rpms it will bog out and back fire. Runs really ruff and will about die unless let off the gas and don't go above 2500 rpms. Seems a clogged exhaust so we unbolted that to rule out that problem, put some new plugs cap and rotor in, tested the egr Valve which tested good, cleaned the throttle body, still same issue. Tested mass air flow sensor and a few others as well, map sensor etc, all testing good.

Kinda at a loss now. No check engine light either. Maybe just time to tell the guy take it to a real mechanic? I hate giving up on stuff, lol

 

[65impalass]

sounds like low fuel pressure. may be fuel pump

 

[jeffcat]

That is what I was going to add. Fuel pumps are good for maybe 125K. Only problem is most all of them require dropping the fuel tank. Here is a diognostic to try. When you start the vehicle and it is cold, when you turn 5he key it should start within two revoluthions. If it needs to crank a few times....you need a pump. Going to be. $200 or more. Get the whole assembly, pump, sender, ect.

 

[Steve@Advance]

Probably low fuel pressure.

Possibly a weak coil.

 

[Bob]

I agree with the others about a possible low fuel pressure issue.

Simple enough to check, though, as there's a pressure check port in the fuel line at the left rear of the intake manifold..

Connect up a fuel pressure gauge with a hose long enough to reach out the back of the hood and stuff the gauge under the wiper facing you and take 'er for a drive.

If fuel pressure drops when it acts up, replace the filter first, before dropping the tank to replace the fuel pump.

 

[1970-1655]

When my fuel pump quit it just quit. Dead on the road.

When you say you tested the EGR valve, how did you test it? Vacuum pump to see if it moves? I had a motorhome with 454 that acted that way so I tested it with the pump and it worked fine. A friend told me to start it up at idle, reach under the vacuum top on the EGR valve and pull up on it. If the valve is good it will kill the engine. (acts like a 3/4" vacuum leak) If the valve part of the EGR is burned, like a valve inside your engine, it will run on without changing RPM or sound. When I pulled mine off the valve in it was burned about 1/4 of the edge gone.

 

[Davidj]

(quoted from post at 02:01:06 02/20/18) Only problem is most all of them require dropping the fuel tank.

Changed the pump on my '02 Chevy last fall by removing the box. If you have a chain hoist or a cherry picker or a loader or 3 friends in decent shape, it's way easier than dropping the tank, at least for an old fat dude like me. BTDT.

 

[tracto-man]

I did a cottage test, made sure it has power, tested voltage while moving it up and down by hand, and while sitting on the engine.

Maybe not the best test? Just what I found on the web.

In my experience with fuel pumps they work or they don't work, but I haven't been around for nearly as long as y'all, lol.

Fuel pressure gauge hooked to the test port shows a steady 60ish psi idling running and while bogging.

 

[tracto-man]

We did test it with the gauge on the test port, remained a steady 60ish psi idling, running, and while bogging out. Will replace the fuel filter anyways today

 

[Determined]

After you change the filter dump some methyl hydrate in the tank.
A cup is more than enough.
Water in the fuel will cause the symptoms you described.

 

[Bob]

If fuel pressure remains near 60, fuel supply isn't your problem.

Try this... disconnect the wiring connector from the EGR valve and see if the problem goes away.

This WILL set a trouble code after a few miles, so the valve will need to be replaced if disconnecting it makes it run better.

 

[buickanddeere]

If replacing an electric intank fuel pump. May want to consider the optional E85 capable pump even though E85 will never be used in the vehicle . The E85 pumps are made from superior materials and will resist gasoline and additives longer before crumbling to pieces inside . .

 

[David G]

I would put a pressure gauge on the fuel line and test under load before going to work to replace fuel pump.

There should be a schrader valve there.

 

[tracto-man]

Checked pressure off, idling, running, under load, bogging, always stays at 60psi

 

[gregww]

My '88 Chevy C 3500 had a problem last year. I thought it was backfiring also at first. Turned out it was popping through the exhaust. It was also exhausting the wet, black, soot out the tail pipe. Made a D*** mess in my garage. I had nothing to do so I started messing with it. Replaced MAP, sensor, electronic pick up in distributor, and a few other things. I figured if I threw enough money at it, I had to fix it right? Wrong. I finally gave up and took it to the dealer. They replaced both fuel injectors, and it was fine. Incidentally, the injectors in the throttle body was going to be the next thing I replaced, but I figured I threw enough money at it already. They were happy to do it for me for around $600. I was using a spoon to throw money, they used a pitchfork. Oh well. Live and learn.

 

[Mule Meat]

May be a little weird to many.

Start it up and try again with a helper. When it starts to act up. Loosen the fuel cap.

Many people are stunned to learn just how big of a role the fuel cap plays on modern automobiles.

 

[David G]

I would say fuel pump is OK.

You really need a scan tool, can pick them up at Walmart for 30-40 bucks.

 

[branderson]

Try disconnecting the wire to the EGR valve. I think it's electronically operated. On the right side of the engine, on the intake manifold. It seems that I had this same type of problem on a 5L in a '95 pickup with a manual trans. It got the hiccups if you accelerated too quickly, if you let out and gradually got into again it would pickup speed, but getting into it too hard would set it off.

 

[Ken in MN]

Unplug the MAF sensor & try it. I've seen 2 over the years that operate well enough to not set a code yet are bad enough to cause drivability issues.

 

[Bruce(OR)]

Generally a low speed miss is the sparkplugs and a high rpm miss is the ign wires. Your 60 PSI sounds like the Vortec engine and those usually run 60/62 PSI. Go out tonight with the dark skies and pop the hood, look for a light show around the plug wires. If you see a light show, you really, really need wires. No light show? Long handle screw driver with a jumper wire from the shank to the vehicle ground.
Hold the handle only and move the shank across the the plug wires and around the plug boots. See a zap? You found a bad wire/boot.
Guess what. Once again, "you really, really need wires."
I don't know what a jumper wire from ground to the screwdriver shank is going to cost or the price of a long screwdriver. I suspect though, they might be cheaper and easier than a pump or much of anything else.

 

[hd6gtom]

I had 1 that was low on power everything checked out OK but still lacked power. I dropped it off at the mechanic and told them to keep it till they found the problem. Fuel pump would put out and hold the correct pressure for quite some time but then it would drop off, might put out the correct pressure for 15 minutes or more then drop off. Might be a place to look.

 

[tracto-man]

Plugs, plug wires, cap and rotor are brand new all ac Delco. Changed the fuel filter today too with no difference.

 

[tracto-man]

Unplugged the MAF sensor, didn't make a difference. Tried same thing with map sensor no difference.

 

[tracto-man]

I'll try that tomorrow

 

[tracto-man]

Tried running with EGR valve disconnected, made no difference, still runs bad under heavy acceleration.

 

[Bob]

Have you checked crank to cam sensor correlation with a scan tool?

This can be corrected by moving the distributor appropriately.

With these engines, a badly worn timing chain can cause "flutter" in the crank to cam sensor correlation and caused "bucking" issues.


Also, you mentioned changing cap and rotor. This is one place to DEFINITELY use genuine GM/AC-Delco parts.

 

[Musky]

Just went thru something very similar in my 93 chev. I replaced every thing electrical I could on the darn thing. Even put a new distributer in it. Took it to a local mech and said I give up. He told me the engine was shot. Since me and that ole truck go way back, I wasn't ready to turn it out to pasture yet. I found a different engine that had less that 75K on it. I bought it and asked the mech if he would change it out for me. Said he would but, took for ever. I told him I wanted to be there when he fired it up. Long story short, it did the exact same thing as the other engine. The mech now pretty embarrassed, said he was stumped and did not now know what to do. His daddy in law( the original owner) stopped, and told him to replace the coil. The mech said it was new, the old man said "change it anyway". Well, it runs great, I drive it every day and love it. Just wish I hadn't popped for the different motor before he talked me into junking the old one.

 

[funky_g00]

From what you describe it sure sounds like a spark issue. Have you checked connections my pickup acted similar and after replacing/checking the entire ignition system it turned out that my entire problem was a loose engine to body ground. The ground made contact just fine at idle but as RPMs increased and engine vibration increased the wire would intermittently loose contact causing a misfire.

 

[4play]

I assume no check eng light on or codes? Monitoring data can tell you if you might have a timing issue (cam retard only should be +/- a couple degrees, ideally 0*), it can also be used to correct ign timing. I have had this problem before and found a worn distributor gear. I have replaced quite a few distributors because of a worn dist gear, some worn to the point it skipped teeth (still running), or they were jumped time enough to not start.

When you had the dist cap off, did you happen to see if the rotor could be turned as in excess play in shaft?

 

[D beatty]

Check cat converter could be plugging up

 

[jacksun65]

Those engines are known for bad intake gaskets. my guess is that when you get up to rpm the vacum increases and you get a lean mix causing your problem.

 

[rbhuntn]

(quoted from post at 22:27:17 02/19/18) Maybe a little OT, but trucks are tool and we use tools to work on them right?

A friend and myself got to messing around with this 1998 Chevy K1500 5.7 350 4x4 pickup. It runs okay but if floored or right about 2500 rpms it will bog out and back fire. Runs really ruff and will about die unless let off the gas and don't go above 2500 rpms. Seems a clogged exhaust so we unbolted that to rule out that problem, put some new plugs cap and rotor in, tested the egr Valve which tested good, cleaned the throttle body, still same issue. Tested mass air flow sensor and a few others as well, map sensor etc, all testing good.

Kinda at a loss now. No check engine light either. Maybe just time to tell the guy take it to a real mechanic? I hate giving up on stuff, lol

I had a 1994 Chevy with the 454 that started doing something very similar and it was the cat converter that was plugged.

 

[tracto-man]

Here's a few screen shots from my $10 Bluetooth scanner. Is that much advance in the timing normal?

Did use all ac Delco parts.

 

[Dune Country]

yes 20 degrees timing at idle is to much, set it static at the timing mark on the pulley.

These engines timings will be taken over by the main cpu as long as you're close. If you go too far outside it will cause intake kickback and backfiring through the throttlebody.

I set mine TDC myself and it took over after first startup and purred like a kitten. And that was just using my finger in number 1 spark plug hole to feel for air.

Also take the distributor cap off and put a wrench on the front crank bolt, rock the crank back and forth and see how far it goes until the rotor moves, more than a few degrees shoes a lose timing chain. (if it has one???)

 

[4play]

Don't worry about that ign timing data, way to many variables factor in to what that reads. I don't know if you can pick other data to look at or not but you need to be able to read Camshaft Retard Offset and see what it says.

If cam retard is reading between 0-2 degrees, you are ok. If it reads anything else the dist is either out of adjustment or needs to be adjusted, which would indicate something is worn.

 

[tracto-man]

Thanks! I'll try looking into that more
tomorrow

 

[tracto-man]

I'll try to see if I can get some more data
from the cheapo scanner tomorrow.

 

[4play]

I forgot to add, if you are able to look at Cam Retard, you need to raise the idle above 1000 RPM, the data might not be reliable under that.

 

[Bruce(OR)]

Since you put the wires in along with the cap/rotor etc, your next free check is the fuel pressure. However, I suspect your coil is to blame. I have found that coils are a shot in the dark a lot of times, I have found bad ones that still met the resistance spec and once aftermarket, hardly fall into the factory spec out the door brand new. The fuel pressure check is key on, pressurize the system and should not drop more than 1 PSI in a minute. Beyond that is a pair of needle nose vice grips with rubber hose over the ends used to pinch off the return line rubber hose. That will isolate the fuel pressure regulator/return circuit. Still dropping? If that is a Vortec engine then the fuel line spider inside the intake might be your problem. It might be possible the newer Vortcs have the ability to replace the fuel lines individually. It has been too long since I played with one of those.
I did see one Vortec that the fuel lines at the back of the manifold had come loose and were leaking. The 2500 RPM makes me suspect either the coil is breaking down or your fuel starved. I would bet on the coil.