Babcotyl

I have a 1992 caprice, originally a 305 with a 350 swapped in by the previous owner.

The problem i'm having is that it's very difficult to rev the engine beyond 1600 rpm. Every time the engine hits this rpm it feels like it hits a wall. O2 sensor readings are all over the place and it only has about 16 in of vacuum at idle. Vacuum lines are all good, including brake booster. It has new intake manifold gaskets and if they're leaking it must be on the valley side because spraying carb cleaner around them does nothing. The EGR is blocked off.

I have a log file of the car idling and going for a short drive around the block. At the end i tested the TPS at WOT and found it was a little off but not enough to cause this issue.

Any ideas would be appreciated. Every thread i can find about these engines seems to be about idling problems that clear up at WOT.

Schurkey

What is the cranking compression of all eight cylinders?
What is the fuel pressure when the engine is misfiring?
How old are the spark plugs? Inspected and in good condition?
How old are the cap, rotor, plug wires? Inspected and in good condition?
Is the pickup coil/distributor mainshaft in good condition?
Have you tested the spark with an HEI spark-tester?
Catalytic converter plugged? Exhaust system crushed?
Cam lift OK? All rocker arms move an appropriate amount?
Cam timing OK? Timing chain not worn-out?
Air filter plugged?
Does the heated-air intake system work properly?

tealman92

Check the fuel pressure and make sure both injectors are spraying fuel.

RBob

iTrader: (1)

Due to the rev-limiting and the low vacuum at idle my guess is on a clogged exhaust. Pull the O2 sensor out and try again. If the situation improves then check for crushed exhaust pipe or a clogged cat.

Note that a cat-con can break up and send chunks to the muffler clogging it.

RBob.

Babcotyl

New: Cap, Rotor, wires, plugs, fuel pump, ignition coil, CTS
Fuel pressure at idle is 12 (controlled by an external adjustable fuel pressure regulator) no gauge in car. Injectors both spray fine. I've tried adjusting this to see if it helps, doesn't change much except becoming loud on cold start if pressure is above ~14psi.

Exhaust is not crushed anywhere and only has a cat (no mufflers). I tried to test for excessive back pressure using my vacuum gauge and found no evidence of that. Vacuum only rises with rpm (up to ~21inHg) and drops to ~16 at idle(~600rpm). I'll try running the vehicle with the O2 sensor out.

I've watched the engine run with both valve covers off and all the valves seem to be opening a normal amount. I also set the lifter preload on one cylinder bank (half turn past no noise while running) and nothing changed. I'll be doing the other one soon.

Haven't done a compression test as this issue seems electronic due to the fact that it occurs at a specific rpm but will consider doing this soon.

Thanks guy.

Babcotyl

The 1600 rpm issue is gone, I think the fuel pressure was a little too low. bumped it up to 14 psi.
After fuel pressure was bumped up i tried running the car with the O2 unplugged and with it out of the exhaust. Neither resulted in normal vacuum.

I logged a longer trip this time with everything hooked up normal.
I just need to figure out why it's not pulling much vacuum.

Schurkey

Seems unlikely, 12 psi should be MORE than enough. GM spec is something like 9--13. You were already on the high side of spec. My K1500 runs perfectly on 11.5 psi; and I only increased the pressure from 10-ish because I had a faulty O2 sensor that was providing false "lean" indications at highway speed. Didn't figure out it was a defective O2 sensor until after I jacked-up the fuel pressure.


You reported 16 inches at low idle, and up to 21 inches with some RPM. Seems fine to me.

Babcotyl

I should mention that the vacuum is not perfectly steady. It fluctuates about +-0.5 inHg normally and sometimes a little more. I set the preload on the other bank with my vacuum gauge hooked up and didn't find anything.
​​​​

Schurkey

Fluctuating vacuum? +- half-an-inch isn't horrible; and depends on the damping of the gauge. In my garage, that thing would get the "Three Sisters" treatment:
Cranking compression test
Leakdown test
Cylinder-balance test.

I'd be looking for one or more "weak" cylinders. This could be from poor valve sealing, poor ring sealing, or other causes. Since labor time isn't a real factor for me, I'd do all the tests on all the cylinders. You might save time by starting with the cylinder-balance test, and then compression-testing and leakage-testing only the weakest cylinders (assuming some cylinders are in fact weak.)

RBob

iTrader: (1)

I missed the implications of this the first go around:

If the 305 TBI unit was retained that explains the rev cut and the improvement with more fuel pressure. In this case 16 psi fuel pressure will increase the injector flow to 61 #/hr (early-mid L05).

Could also be that a mild cam was installed which would account for the low idle vacuum.

RBob.

henryd3

iTrader: (128)

I assume the prom and injectors were changed to compensate for the increased displacement...

Babcotyl

It has 350 injectors and prom. I think the ESC module may be from a 305 but couldn't find the part number online to check.
I'm redoing a few grounds tomorrow and if that doesn't do anything I'm going to do a compression test I guess.

Babcotyl

Also, i was told it had a mild cam but I can't tell if it does because it runs like trash at high rpms...

Schurkey

Calling it a "mild" cam is deceiving. The cams that come with TBI engines are mild. What you folks are talking about is a LESS-mild cam; one that will alter vacuum levels enough to throw the calibration of the fuel and spark tables out the window.

I can't help you there. My K1500 runs perfectly fine with a bit of extra compression, and a change from a TBI flat-tappet cam to a TBI roller cam--but the cam is still TBI-mild and no real performance boost.

'Course, the "mild" cam could easily have been installed improperly, leading to the cam out-of-synch with the piston motion. A tooth "off" on the timing chain is easy to do for a beginner. This will show up as improper compression pressure when you do the compression test; and if it's bad enough, it can signify bent valves.

Babcotyl

I'm away from the car for about 3 weeks but intend to do a compression test first thing when i get back (I've been avoiding it in fear of bad results ) . Thanks for the ideas guys. I'll update this thread with the results in about 3 weeks.

Babcotyl

I did the compression test today. Results are:
1- 155
2- 160
3- 164
4- 165
5- 160
6- 165
7- 153
8- 165

RFmaster

Babcotyl

Cranking compression within 10 PSI is normal. 160 PSI for stock 350 is reasonable.

I would check and make sure that base timing is set correctly.
For stock LO5 it is 0 deg with EST bypass connector open. Disconnect EST single circuit plug tan/black wire, start engine. Adjust timing, tighten your distributor hold down bolt, shut engine off, disconnect battery to clear ECM for set EST code. Reconnect EST bypass connector, restart engine. At idle, 650RPM timing will be bouncing between 16 to 20 deg.
If camshaft, timing cover, or balancer were (was) replaced without verifying TDC base timing is likely be off. That may explain engine lazy high RPM performance.

//RF

Babcotyl

Well it's spring break but too cold to diagnose this. I did a log because it was surging horribly at idle. It seems to happen when i decrease the fuel pressure too far.

Question: what is the crank pulse thing in the log file? should it always be yes and what sensor causes it?

log file is attached

Thanks.