Perplexing Loss of Power...

[pfloro]

Hello Everyone:

Sorry in advance for the long post but I wanted to provide as much information as possible.

I’ve had a nagging drivability issue with my 307 V8 powered ’84 Toronado for quite some time. At any vehicle speed, opening the throttle beyond about 2/3 ‘flattens out’ the engine. RPM doesn’t increase, power drops & the engine roughens up a bit. Further opening the throttle doesn’t change anything. This engine has 188,000 miles under it’s belt.

Information & tests:

Cold or hot start, idle, take off, mild-to-moderate acceleration & cruise are fine. I have rebuilt the CCC Q-Jet several times over 25 years and everything is set using factory procedures (all tamper caps have been removed). All vacuum hoses & ‘T’s are new and vacuum sensors are good. I have selectively capped off each vacuum driven device (EGR, PCV, canister purge, etc. ) at the carb with no improvement. Carb is tight to manifold too. I have even temporarily taped the secondary air valve closed thinking this issue might really be a ‘bog’. It doesn’t seem to be. Once the engine ‘flattens out’ under load, it stays flat until I let up on the throttle a bit. Based upon when the secondary throttle plates open in this Q-Jet, I know this issue occurs while they are still closed. All the CCC sensors are almost new and no error codes are being set. Manifold vacuum is a vibrating 18” at idle (2600’ elevation). I understand that a vibrating vacuum gauge reading indicates worn valve guides.

I’ve run tests in each manual ‘gear’ with the torque converter clutch locked out. I've had a switch installed for years which prevents the TCC from locking (useful for hilly terrain). This issue is independent of the ‘gear’ with or without TCC lockup. I should add that if I gradually increase the throttle opening in any gear, rpm builds up & the flat spot does NOT occur. It appears that if a load is placed on the engine too quickly, the flat spot occurs.

I just replaced the AC R46SZ plugs with the same. Old plugs looked fine after 20,000 miles & were gapped @ .080” (factory spec). I tried re-gapping the new plugs @ .060”. No change… Timing is set to 20 degrees BTDC (factory spec). Wires are fairly new & pass resistance test. Cap & rotor are almost new (no visible carbon tracks). I just swapped in a known good Delco ignition module. No change. Pickup coil is also fairly new. Also just swapped in a known good Delco ignition coil. No change. I even hooked up an old fashioned neon timing light & taped the light to the windshield. I wanted to see if the spark degraded when the problem occurs. No change in the intensity of the neon flash was observed.

The original timing gear set was replaced at about 150,000 miles with a Crane Cams double roller set. The engine burns no oil but I have not yet performed a compression test… If compression was low, I think (IMHO) I would be experiencing other symptoms as well.

I have monitored the Mixture Control Soleniod ‘duty cycle’ using a dwell meter while driving. Everything looks good (it floats about the 30 degree mark while in closed loop at idle). The ECM changes the duty cycle in response to different driving conditions. I believe the primitive CCC system is doing it’s job. Emissions are low & fuel economy hasn't changed. I even rigged a switch to disconnect the MCS which provides an unrealistic rich mixture. This didn’t cause any change in the loss of power.

This problem sometimes becomes a safety issue when there is a need to merge into traffic, etc.

Your thoughts are greatly appreciated…

Thanks,

Paul

[earthworm]

There is too much air and not enough enrichment, causing the flatness.
Is there any adjustment for the acceleration pump?
I once owned a '80s Chevrolet Citation I4 with, as I recall, hesitation.
A mid range adjustment was set at the factory - too lean.
But the running quality improved at the 100K mark, for some reason..
Also, check your vacuum advance and retard, if any.
Good luck, and success

[DeSoto Frank]

Sounds like fuel starvation ?

Have you tested the fuel pump for pressure and volume ?

Could also be clogged vent system for the gas tank; try running it w/o the gas cap and see if that makes a difference.

Are you sure the fresh-air plenums to the air-cleaner are clean and free of obstructions ?
( I had a "weird" problem with an '85 Chrysler Laser that turned-out to be a squirrel's nest built in the fresh-air intake in the front fascia - the last place I thought to look for a performance problem; when I pulled the cover off the air cleaner and found some fluff and acorn shells, that clued me in ...)

At 188,000 miles, I wonder how much wear & tear have affected the camshaft...

How about a clogged catalytic converter ?

[kaycee]

I had a similar problem with my '85 Riviera with the 307 Olds engine. It turned out to be a blocked catalytic converter. You may want to pick up a bypass pipe for the converter and see what happens, even though emissions seem to be minimal.
kaycee

[pfloro]

Thanks for all the quick replies. Some additional information:

1) A 'Little Red' Holley electric fuel pump replaced the mechanical one some time ago. I don't know if the problem existed before the changeover. This electric pump is mounted on the frame near the tank. It is internally regulated to 7 PSI.

2) The fuel tank & sender are only a few years old and all lines/hoses have been replaced. Fuel filter is fairly new too but could still be clogged...

3) The catalytic converter was replaced about 5 years ago. Of course this doesn't mean it's not clogged.

I can continuously cruise @ 70+ MPH without any trouble. Only when I try to quickly raise RPMs does it 'flatten out'. I'm planning to replace the exhaust system very soon as the muffler has 'blown out' internally & I don't like hearing that exhaust drone in a luxury car! I've replaced the exhaust system from the cat back several times before & have looked into the cat's honeycomb. Would I be able to see if any blockage exists?

Since I can read the float level (through the Q-Jet air horn slot) when the car is stationary, is there any value to measuring the level while keeping the RPMs at say, 2500? This might show if the fuel level in the float bowl is not being maintained at the set level... (?)

I'll put my Sherlock Holmes hat back on, keep investigating & report...

Thanks,

Paul

[DeSoto Frank]

When did this problem appear ?

Did your area switch to alcohol-blended gasolines before you started having these problems ?

[pfloro]

Hi Frank:

It's hard for me to pinpoint when this problem started. However, metro Tucson, AZ has required E10 for 5+ years. I know this problem doesn't go back more than a year or two.

On my way home today, I selected manual 'gear' range '2' just after rolling. I could gradually accelerate to 50 MPH while the engine RPMs easily increased. I must have been hitting close to 3000 RPM. This engine speed was maintained for at least 15 or seconds (traffic forced me to slow down). If there is a restriction in the exhaust system, would I have been able to spin the engine up to this RPM? The flat spot issue under load always occurs way before the engine reaches, I would guess, 2000 RPM. I haven't tested with a tach connected.

Thanks,

Paul

[DeSoto Frank]

Paul,

Good test; 15 seconds may not be enough time to provoke fuel starvation.

If the exhaust were clogged, the engine would probably wind-up initially, then the RPMs would drop as back-pressure builds.

If you can get out on some lightly-travelled roads and try running the car for a mile or so at 40-50 MPH in D2, and see if it wants to lean-out.

I have a starvation issue with my '41 De Soto that began about 4 months ago - it only occurs during long pulls at high speeds - such as the five to nine-mile grades on the NE Penna Turnpike Extension... doesn't happen on my local sprints up & down I-81 to & from work.
It shows-up gradually, usually about 1/2 way up the grade.

Try connecting a vacuum gauge with enough hose so that you can read it from the driver's seat, and run the car, noting vacuum reading on the flat at 55 - 60 mph, then advance the throttle gradually, noting the vacuum reading ; I wonder if you'd see a sudden drop in vacuum when the engine bogs ?