Bad Stumble/Miss/Backfire 1995 Suburban

[Q-Bert]

Background:

1995 Chevy K1500 Suburban
5.7 engine, 110,000 miles

Problem:
Couple weeks ago started with a stumble and miss (no backfire). Got worse as a few days went by, and would not accelerate well at all.
No SES lights.

Work performed:
Replaced plugs, wires, cap, rotor, coil, PVC, air filter. All AC/Delco parts.
Old plugs looked fine, but gap had increased to .045.
New plugs gapped at .035. Miss/Stumble continued.
Replaced fuel filter. Same problem.
Checked fuel pressure, 11psi

Read thru nearly 60+ pages on forums.
Removed and inspected original IAC. IAC was fouled up, replaced IAC.

With new IAC, now engine has a significant miss and backfires. Engine tends to idle high (approx. 1200 and very slowly comes down to about 900). To get the vehicle to move at all without stalling the engine I have to continually feather the throttle and I can get it to about 35-40mph max. Once the vehicle gets up to about 35, I can let the vehicle coast on idle. If I touch accelerator to get some more speed engine will miss and stumble and backfire if I push it too hard.

Any ideas? Thanks!

[maxwedge]

Welcome to AF. Is this a Vortec or TBI, if Vortec the fuel pressure is too low, if TBI check the fuel pressure with the engine under load.

[Q-Bert]

Originally Posted by maxwedge

Welcome to AF. Is this a Vortec or TBI, if Vortec the fuel pressure is too low, if TBI check the fuel pressure with the engine under load.

Thanks for the welcome, I've been a member for couple years, just never posted before
This is for TBI, fuel pressure quoted is with A/C on, trans in drive.

[maxwedge]

Get it under load and see if it maintains the FP. This all asumes the fuel is not contaminated. If the FP is good under load, make sure both injectors are spraying fuel.

[Q-Bert]

Originally Posted by maxwedge

Get it under load and see if it maintains the FP. This all asumes the fuel is not contaminated. If the FP is good under load, make sure both injectors are spraying fuel.

Thanks. I assumed that with the A/C compressor engaged and the trans this is considered "under load"? A/C and trans in drive seriously affect the idle. I'll try hooking up a fuel pressure gauge tomorrow with a hose long enough so I can watch it while I'm trying to drive it.

Both injectors appear to have a smooth conical pattern. No dripping when off.

[Q-Bert]

Under "attempted" acceleration, engine in drive, A/C on (engine will continually miss and backfire) fuel pressure never drops below 10psi. It sits between 10-12psi.

[2000CAYukon]

Fuel pressure looks good for TBI. Does the MAP hold vacuum?

If you remove the 02 sensor and drive it, does it accelerate better? Cat or exhaust could be blocked.

Do you access to a scanner?

Check all vacuum hoses for cracks and loose connections.

Does the PCV valve fit snug in the valve cover?

//2000CAYukon

[Q-Bert]

Where is 02 sensor on these? Exhaust manifold or further back? Passenger or driver side? I'll try to remove it. Somehow I doubt exhaust is restricted because I was able to get it going about 40-45mph today with no miss/backfire and I can get it to rev up to 3000-3500 in park. Acceleration was another story though. Took me a while to get up to that speed.

I had done some more troubleshooting today disconnecting and reconnecting some sensors.

The MAP appears ok. It's 4.8 with engine off, and about 1.8 with engine running. Since I found that the engine "sometimes" runs better with the coolant temp sensor disconnected. Engine idles at about 1800 with it off, which is understandable since PCM thinks engine is cold and throws a SES light. I checked resistance of sensor and it appears correct vs. a table I found. The disconnected CTS I think is just masking the problem when the engine tries to idle at it's normal ~500. I can throw car in gear and car will eventually "idle" at 1500rpm up to ~30mph. Once I start touching the accelerator though (either at a stop or while "cruising" at 1500rpm idle), the engine will stumble. Is PCM supposed to advance timing upon acceleration? Does this thing have a pickup coil in the distributor? Is it testable?

[Q-Bert]

bump?

[Q-Bert]

Engine runs the same with O2 in place or removed. I'm running out of ideas.

[Q-Bert]

Took it to a friend. He says it's a loose/worn/jumped timing chain.
Does this make sense? Wouldn't I get some kind of ""camshaft to crankshaft correlation fault" come up with SES light? Or do the 95s not have these sensors?

[Q-Bert]

If the timing is checked with the "set timing connector" connected, will a timing light show the timing jumping all over the place? I'm assuming the "set timing connector" needs to be disconnected to check timing?

[2000CAYukon]

Yes, set timing connector should be disconnected to check timing.

If you think the timing chain jumped a tooth, then I would rotate the engine until the #1 cylinder is TDC (compression stroke) using mark on the balancer. Then pull the valve cover and make sure both valves are fully closed. If either valve is not closed, the timing chain could have jumped.

I have also seen the balancer go bad and the mark will be incorrect (outer ring has spun due to the rubber going bad).

The idle is too high and I assume the TBI gasket has been replaced as that is a common problem with the TBIs (original gasket was too thin and has been redesigned). Any vacuum leaks can cause a high idle.

A compression test may also tell you if the timing chain is off.

//2000CAYukon

[Q-Bert]

2000CAYukon, thanks.

I'll check the timing and check for leaks on the gasket.

Thanks again.

[v8_ranch]

any luck yet?