Stumble at idle

BMWSTEVEN

MEMBER
1993 CHEVROLET 1500
5.7L
V8
4WD
AUTOMATIC
222,222 MILES

At idle in park or drive, cold or hot, the engine will stumble and intermittently misfire. The engine is a GM dot r crate motor with about 15,000 miles on. The fuel pump replaced with engine five years about old. The distributor is brand new from GM complete assembly. The throttle body rebuilt with new gm injectors. IAC replaced. EGR replaced. Map swapped no change. TPS swapped no change. Timing is correct to 0. Wires and plugs AcDelco. Checked and added extra grounds from battery to frame, frame to cab, cab to engine. Check grounds on engine for engine computer replace and solder one. The other looks okay. Check voltage drop of engine and resistance all in acceptable ranges..03 ohms. And.01 volt drop. Fuel filter replaced. Fuel pressure at 12 psi. Smoke intake no leaks. PCV valve blocked or and or switched doesn't change engine still stumbles. No play in throttle body linkage or shaft. New o2 sensor. Replace IAC connector and injectors connectors. Block learn right at 119 to 128. Map sensor TPS sensor voltage all in spec. T stat new engine at 195. I am about out of ideas. Thinking the ECM is to be my final issue. Oh, also did a secondary scope no particular cylinder stood out as a problem but did notice the average kv seamed low for an hei system. Around 5kv. Oh the coil is AcDelco new too. As well as the short harness that goes between the coil and distributor. And she passes smog no issues. And runs great off idle and while driving no power loss.

3 Replies

JACOBANDNICKOLAS

MECHANIC
108,179 POSTS

Welcome to 2CarPros.

It sounds like you have done everything. If you have a timing light connected, do you see advance when you first throttle the engine? Also, and don't laugh, I dug up a technical service bulletin (TSB) related to a hesitation. It relates to the plug wires. I realize you have gone through and check or replaced them, but I thought I would add it. You know, even a bad knock sensor can be causing a delayed ignition advance.

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Number: 93-35-6D
Section: 6D
Date: OCT. 1992
Corporate Bulletin No: 716404R
ASE No: A1, A8

Subject:
ENGINE MISS HESITATION OR ROUGHNESS DUE TO PIERCED SECONDARY IGNITION COMPONENTS

Model and Year:
1980-93 ALL PASSENGER CARS AND TRUCKS

THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 87-121, DATED MAY 1987. THE 1989-93 MODEL YEARS HAVE BEEN ADDED. ALL COPIES OF 87-121 SHOULD BE DISCARDED.

During the diagnosis procedure for an engine miss, hesitation or roughness, a spark plug or spark plug wire condition may be suspected. Several types of commercial or homemade diagnostic equipment required the secondary ignition boots or wire to be pierced. This is normally done to check for spark plug firing or to perform a cylinder balance test. Similarly the use of pliers or other such tools to disengage a spark plug boot may pierce or damage the boot or wire. Secondary ignition components should not be pierced for any reason.

Piercing a spark plug wire and/or distributor boot may create a condition that will not be immediately apparent. Over time, the hole in the pierced boot may allow a ground path to develop creating a plug misfire condition. Heavily moisture laden air in the vicinity of the pierced boot may accelerate this effect.

Piercing a secondary ignition wire creates a gap in the wire's conductive core. This gap is a point of high resistance. The current flow in the wire will increase to compensate for the higher wire resistance. Over time, the wire may fail creating a plug misfire condition. The time required for the condition to appear depends upon the extent of damage to the conductive core.
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Next, there is a TSB related to the spark plugs. Because of a new design used on your vehicle, the insulator is slightly longer. What was happening was the insulator was cracking because typical plug sockets weren't deep enough. Here is the TSB.

Subject:
NEW DESIGN SPARK PLUGS

Model and Yew:
1991-93 ALL PASSENGER CARS AND TRUCKS WITH GASOLINE ENGINES

This bulletin cancels and supersedes dealer service bulletin no, 91-234a-ob, dated july 1992. The 1992 and 1993 model years have been added. All copies of 91-234a-ob should be discarded.

In 1991 GM introduced a new design spark plug for use in all trucks equipped with gas engines. These new design spark plugs have a ceramic insulator which is approximately 1/8 inch longer than the insulator used in previous model years.

The longer length spark plugs, which conform to S.A.E. And I.S.O. Engineering guidelines, magnify the problem of cracked insulators because currently, most spark plug sockets are not of sufficient length to properly engage the shell hex. If the spark plug shell hex is not fully engaged in the spark plug socket wrench, the socket may cock at an angle and cause insulator cracking and/or breakage during plug installation or removal.

When servicing these new design spark plugs, make sure that the spark plug socket being used is deep enough to accommodate the longer length insulator. The spark plug socket wrench should conform to the proposed S.A.E. And I.S.O. World standards for spark plug socket wrenches. Spark plug socket wrenches that conform to these standards are designed to accept the lengthened spark plugs and allow full engagement of the hex nut on the shell of the spark plug.

Use of a spark plug socket which is NOT deep enough may result in the ceramic insulator becoming cracked above the spark plug shell.

Note:
Some cracks in the insulator m" not be visible. Such cracks may later cause a spark plug to misfire. Spark plug misfires are often misdiagnosed as a slipping transmission, defective torque converter clutch, engine imbalance, or malfunctioning fuel system.

To prevent insulator damage, it is recommended that the proper spark plug socket wrench be used when removing or replacing spark plugs. One such spark plug socket is the Kent-Moore J-39358 spark plug socket.

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I am trying to find things that are not normally considered. Let me know if this helps and also let me know if you see advance when throttling the engine when you have a timing light on it.

Joe