TBI stumble under load. SOLVED!!

[fancyTBI]

This is a long post, so please bear with me!

Details
1992 C1500 5.7/MG5
Originally a 4.3 now a 5.7 (I did the swap, have had the truck since 1998, 323K on the original 4.3 and it still wasn't dead!)
The 5.7 has a claimed 96K miles on it and is a roller cam 5.7 from a Caprice.
I did this swap as best as I could. I wanted it to look and act factory, so it has the correct ECU, prom, injectors, and other misc. No headers, stock manifolds, full exhaust and O2. It does not have a converter.
I have a rough idle, stumble under load, and I believe a misfire somewhere.
Driving in gears 1-3 are bearable but in 4 and 5 there is little power and the truck surges, stumbles, and shakes awful. Those gears are almost unusable.

Already checked/changed
Did a compression test, all cylinders and they were consistent at 190psi. Seems like most compression tests should yield around 150 psi. Could this be an issue?
I've tried 2 different distributors. One was new from Car Quest, the other was the original from the 5.7. The original one worked a lot better and cleared up some issues.
It has a new cap and rotor on it now, it doesn't look like it is arcing, and the wires are good.
Timing is set at 0*, new timing chain and it was installed correctly.
Re-set valve lash while the engine was running with a 3/4 turn.
There was little to no carbon build-up on the cylinders when I took the heads off. Heads were good, no cracks.
New fuel filter, fuel pump works great. It's not a fueling issue.
Most important: NO CODES! So that doesn't even help.
The ECU is checked and working properly.

At a loss here, I cannot think of what else it could be.

 

[redfishsc]

If it's a roller cam 5.7 from a Caprice then the engine is likely a Vortec type.... was this originally a Vortec? Did you retrofit a throttle body/intake onto it? I'm not sure that the Caprice ever got a throttle body type that also had roller cams but obviously I could be wrong there.

Either way, you sound to me like you either have bad fuel pressure coming in, or you have bad spark timing. I can't remember what fuel pressure a TBI should be running at. I've had several fuel filters get clogged on me from a bad tank of gas, and it caused EXACTLY the same symptoms--- ran like a turd at any load.

I've also had (and.... currently have ) distributor caps get fried and cause this. Even 4 month old caps.


190 is great compression, I've seen 185 on several L31 vortecs with 200K+ miles.


Just know, if this were a Vortec anything like the L31 vortecs that come in 1996+ trucks, there's a good chance you really are fighting timing. The L31 Vortec's timing was basically not adjustable. The only thing you had control over really was a very tiny twist of the distributor to adjust cam retard, literally only a couple degrees of turn.

 

[fancyTBI]

It's definitely not a Vortec like the trucks, nor is it an LT1.

When I purchased the engine in 2016, it came with a throttle body, caprice air cleaner box, and it has the "standard" 193 swirl heads on it. It might be a 1993, but it is definitely a LO5 engine.

I can try and replace the fuel filter again, the gasoline in it was old, but I filled the tank before I drove it to help clean it out. I've never had this problem before with old gas (sometimes it sits for 6 months or more at a time). They are inexpensive and it is worth trying again.

I've also heard people getting bad caps from all kinds of part stores. It could be that as well, you are right. I'll check fuel pressure, I hadn't done that yet since before the engine swap it was perfect. And wouldn't a lack of fuel cause it to be hard starting? Mine fires right up first try, no hesitation there.

Thanks for your response!

 

[evilunclegrimace]

Until you verify the pressure don't assume that it is not a fueling issue. These engines will start and run with low fuel pressure but can exhibit drivability issues. Fuel pressure is 9-13 psi. Check your EGR, Disconnect the vacuum line and plug it. Take it for a test ride to see if it goes away. I just had this same problem with my 92 K 1500. It had a stumble under load and I had a heck of a time finding the cause.

The EGR acted normal at idle, I could lift the pintel off the seat and it would induce a leak and cause the engine to miss/run rough like you would expect. I ran all of the diagnostics and every thing checked out. It drove me nuts for a while, so, I fired up the parts cannon and got lucky with the first volley, evidently the EGR was sticky and would not operate smoothly. With a new EGR the problem disappeared and it runs fine. Maybe this will help. good luck.

 

[fancyTBI]

@evilunclegrimace I don't think it is a fuel issue. It starts great and in gears 1-3 if you really romp on it it will get up and go. So I believe it is getting enough fuel, never had this issue before. Fuel pump is not that old and was replaced in 2012 with a new Delphi unit. It also was getting enough fuel before the engine swap (engine was only out for a month, the 4.3 before it ran great no issues).

As far as the EGR, that is out of the equation. I forgot to mention that I blocked that off and plugged the vacuum line. No code is thrown either.

It has a stock prom chip in it now, but I do have one that is meant for a stock setup with the EGR unhooked so no code is thrown. But I had this issue with the custom chip so I thought maybe it was a bad tune.

I feel that it is something ignition related.

 

[Schurkey]

1. Roller cam LO5 in autos was common in the early '90s. I put one in my K1500 almost 20 years ago. The Caprice LO5 has flat-top pistons instead of the K1500 LO5 dish pistons. Ran flawlessly on the stock computer and PROM.

2. You don't have an EGR, and it's not setting a code? That's a problem right there.

3. 190 psi compression seems really high. Sure the gauge is accurate? Having the cam timing overly-advanced could do that, too. I test my compression tester gauges now and then by connecting them to shop air pressure. One compression gauge can be compared to the gauge on the pressure regulator. Two compression gauges can be compared to each other AND the gauge on the pressure regulator.

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4. I'd still want to know the actual fuel pressure UNDER LOAD. A weak pump could supply appropriate fuel pressure at idle, but not supply enough during heavy throttle, high RPM.

5. Connect a scan tool, look at the data stream. The answer is almost certainly in the data stream.

 

[lickmyburger89]

I'm with the other guys, don't rule out a fuel issue just yet. Even if fuel pressures are still good, sensors sending the wrong info will make her run like crap. Your issue is similar to what i'm experiencing but I've checked everything except for the ecm. i'd check the coolant temp sensor and map sensor and go from there. It really wouldn't hurt to check fuel pressure though. I've seen new pumps go out in less than a year

 

[fancyTBI]

@Schurkey, under load at high RPM in gears 1-3 it does not stumble. I can rev it up to 3500 or more and it acts and shifts fine (lots of get up and go!), but anything under almost 2K rpm is bad. Before the engine swap the fuel was fine, no stumble, good idle. I can test fuel pressure this weekend.

The EGR is blocked off and the solenoid vacuum lines are plugged. However, it did throw the code once when driving it. This is how it was done before and I did not have a SES light. Now recently driving there is no code. I assume because the lines are plugged and it senses vacuum.

I put a new timing chain on it and had the cam and crank marks lined up to 0. Cylinder 1 was at TDC. So no advance or retard. Chain is tight, doubt it slipped.

@lickmyburger89 I was going to unplug the map sensor tonight to see how it acts, good thinking. I also wonder if the ECU is getting a bad reading from the sensor in the intake for coolant. It seams like it runs poorly cold or hot. But the brand new one in the head was bad. Guess I shouldn't rule out the other.

 

[lickmyburger89]

I'm actually dealing with a somewhat similar issue. been chasing it down and everything i've tested has come out in spec. you'll probably ave to test a lot of what i did. i'm going to try and get some time today to double check wiring on the truck and make sure i'm not getting any shorts.

what i've checked on my truck: MAP, CTS, IAC, TPS, oil pressure sensor/switch, o2, injectors, fuel pump relay.

i've also changed a weak fuel pump, filter, and rebuilt the leaker throttle body

 

[evilunclegrimace]

@lickmyburger89 I was going to unplug the map sensor tonight to see how it acts, good thinking. I also wonder if the ECU is getting a bad reading from the sensor in the intake for coolant. It seams like it runs poorly cold or hot. But the brand new one in the head was bad. Guess I shouldn't rule out the other.

The temp sender in the cylinder head has nothing to due with how the truck runs. It is for the gauge only. The Engine Temp Sensor in the intake is the one that the ECM uses to make adjustments. Make sure that the ECM can recognize a fault code, tap the block near the knock sensor and see if the ECM sets a code or unplug the ECT in the manifold and see if the ECM sets a code. try a tap test on the ECM and see if it changes anything. These trucks can start and run with a faulty ECM.