1969 Chevrolet Corvette - Wear & Tear

'84-'96 Corvette Tech

Chris Petris Nov 1, 2007 0 Comment(s)

Lethargic LT-5
A number of people have looked at the ZR-1 Beast and a lot of new parts have been tried with no success. The symptoms are it will cough or hesitate while at idle rpm (650ish) and increasing the throttle quickly. Snap open the throttle, and it will cough and hesitate, hot or cold. Engine temperatures make no difference. If I step on the throttle quickly as in revving the engine for an off-the-engine start and clutch engagement, or as in down shifting where a double-clutch technique is used, the engine will cough and hesitate. I've had it to Chevrolet garages twice and to local Corvette people, and no one has been able to fix it. A number of code scanners have been tried, but no codes appear. The engine runs strong in the top end, and my complaint is in the low-end response. My basis for comparison is my '96 LT-4 that is very crisp and clean when operating as described. The LT-5 is lethargic and, frankly, a disappointment compared to my LT-4.

Some of the new parts I've tried are: a current GM E-PROM, plugs (AC gapped per specs), ignition wires, coil packs, fuel filter, fuel pumps (both of them), injectors (new primary and cleaned secondary), O2 sensors, vacuum lines, and so on. Nothing I have tried has eliminated this problem. The plenum has been off numerous times as I looked for vacuum leaks and checked the performance of the various sensors. I even tried changing the complete ECM (computer) with no change.

These symptoms act like a carburetor that has a surge pump not working. That is, advancing the throttle with the mixture too lean, the engine will cough. The scanners look for a code but for an instantaneous condition, where the mixture is not correct. my guess is that the computer does not report this condition. Any thoughts?
Al, Seattle

I agree from your explanation that the problem may be related to fuel enrichment or late spark timing. LT-5 engines don't have good torque at low engine speeds, and they really come alive as engine rpm peaks. Your LT-4 has much better low-end grunt and will feel better than the LT-5 until the LT-5 engine rpm builds. Don't expect the LT-5 to leave the line like the LT-4 unless you make some serious mods to the LT-5 or change the differential gear set to get better leverage off the line.

My first thought would be the spark plugs. The distributor-less ignition systems seem to be touchy about spark-plug condition. My next thought would be the coil packs, but you've changed all these parts and more to no avail.

At this point, I would check the MAP (manifold absolute pressure) sensor for an oil-filled vacuum hose. LT-5 engines are built to run at high rpm, as we mentioned earlier, and tend to have crankcase blow-by. The blow-by allows engine oil to pool in the MAP sensor vacuum supply hose. When this occurs the vacuum signal is slow to react, and both ignition timing and fuel follows suit. Many LT-5 engines we've seen have the oil-filled MAP hose, which we clean out and tell the customer to check periodically.

The next area to look at is the secondary injector control system. If the secondaries are coming in too quickly, the engine will bog down, and performance will suffer. If the secondaries open immediately from the crack of the throttle, you are trying to stuff a whole lot of air into the engine at one time. The engine idles on the small center throttle plate in the throttle body. Then the main throttle plates open up. As engine rpm increases, the secondary plates in the runners open. Hopefully, all this occurs in smooth progression.

A few years ago, we had an LT-5 problem with poor performance because the secondaries weren't opening at all. The secondary throttle-control module behind the battery had gotten moisture in it and corroded it. The corrosion made the secondary throttle response erratic. Does the engine cough when you get on it with the valet key in valet mode? Obviously, power would be down because the secondaries should be locked out and the air flow limited. If the cough still occurs with the key in valet position, it could mean that the secondaries aren't controlled. The top-end performance should be the same as if the switch was in full power mode because the secondaries are open all the time. You can check the condition of the secondary control box once you remove the box from the firewall. The cover can be pulled off to see if corrosion is occurring. The secondary black box is rectangular-about 3-inches tall by 7-inches long and approximately 1-inch thick. This should point you in the right direction.

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