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350 TBI idle misfire only

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87sub91

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Hi there, I am new to CK5 and I am hoping you guys can help me out with my 1991 4x4 Suburban. The problem that I seem to be having is the truck misfires at idle only. You can hear it in the exhaust and feel the engine shake only when it is idling, put it in drive and it goes away. Drives great with good power and gets decent gas mileage. The truck has the stock L05 350 TBI with 192,*** miles. I have replaced just about everything once or twice. The things I have done to the truck are (new intake gaskets, plugs, wires, dist. cap+rotor, distributor, TBI rebuild, CTS sensor, knock sensor, and map sensor.) I've replaced all the vacuum lines, thinking that the miss was caused by a vac leak, no fix. I have yet to find a vac leak anywhere. I'm just about fed up with this thing.:dunno: Any help will be gladly appreciated, Thanks! Oakley
 
First thing is narrowing down your problem. As always, check any recent work you've done. When I get something that acts up after work has been done, that's the first place I go to check it.

Any codes? They can provide a useful starting point.

Make sure that you're really having a misfire and not a physical problem. Shake at idle can sometimes be bad motor or transmission mounts, because at idle is when the engine will shake the most. When I had a bad mount, it shook at idle, smoothed out during cruise, and would bang the exhaust during transition from load to no load. 192,000 miles is a lot. It would be worthwhile to check compression and leakdown, if nothing else to eliminate an engine problem.

Inspect all your plugs and plug wires. Wires can cause a lot of intermittent issues if there are current interruptions or bad connections. I had a wire that was to close to a header and became damaged. It caused a miss at idle and under low rpm load, but no other time. Check your whole ignition system, to include timing, and make sure you've got good spark.

Put a vacuum gauge on it and check to see what you're getting. Low or fluctuating vacuum can indicate a leak. Additionally, a gauge can help you spot other issues like valve train problems, exhaust obstructions, etc. It wouldn't hurt to go around your lines and spray some brake cleaner, or use a propane torch (not lit!) to see if the idle changes. Vacuum leaks can be tough to find, especially on FI vehicles, since they will do everything they can to adjust, and can mimic misfire conditions. Does the truck idle higher than normal? It's not common, but brake boosters do sometimes leak vacuum as well.

If you say it runs well and has good mileage, it's likely a minor issue. Start tackling each system (fuel, air, spark) and eliminating potential causes. Good luck!
 
First, foremost, before you do anything else: get an ALDL reader - I recommend Red Devil River's Bluetooth unit.

Without the data afforded by the ALDL, you're just mostly guessing.
 
I had this exact issue, I replaced the distributor and coil, the thing that lead me there was misfire from the injector not firing, this process in reverse is such..

The ECU waits for a pulse from the distributor, this in ALDL is called a DRP or distributor reference pulse. once the ECU sees this it can coordinate the injector pulse with the spark. now if you have intermittent issues like i did for a really long time this will drive you nuts, aldl does not know if a misfire is occurring, there is no feedback in a TBI ecu to know this, it always assumes the fuel and spark got there.

So check these items first,

Injector wiring, wiggle pull and shack the wires from the harness to the injector, for me I had an issue with the plugs not staying seated (even new ones) so I ended up tweaking the contact inside to make sure it was connected.
Tach wiring from distributor to ECU, make sure this isn't being touched by a plug wire, inductance causes all sorts of weird behavior.
Distributor shaft play, I happened to pull mine out and the shaft would barely spin, this was a reman from Napa. It lasted 18 months. If the shaft touches and grounds out the magnetic reluctor cannot accurately transfer spark, cross firing happens.. this will show on the rotor and cap.
My coil went bad only when it was warm, cold start was good.. saturated coils only rear their heads when temp is an issue.
one way too identify its a bad coil, disconnect the spout plug(timing control for ecu) and if it still acts funny this means its not coming from the ecu, the spark is controlled only by the distributor and coil at this point. This in my opinion is the only way to accurately test those two components.
 
Another thing to check for is an exhaust leak around the exhaust manifold area. Exhaust leaks close to or at the exhaust manifold can cause backfires in the exhaust system when cold air ignites un-burned fuel in the exhaust system.
 
sreidmx said:
I had this exact issue, I replaced the distributor and coil, the thing that lead me there was misfire from the injector not firing, this process in reverse is such..

The ECU waits for a pulse from the distributor, this in ALDL is called a DRP or distributor reference pulse. once the ECU sees this it can coordinate the injector pulse with the spark. now if you have intermittent issues like i did for a really long time this will drive you nuts, aldl does not know if a misfire is occurring, there is no feedback in a TBI ecu to know this, it always assumes the fuel and spark got there.

So check these items first,

Injector wiring, wiggle pull and shack the wires from the harness to the injector, for me I had an issue with the plugs not staying seated (even new ones) so I ended up tweaking the contact inside to make sure it was connected.
Tach wiring from distributor to ECU, make sure this isn't being touched by a plug wire, inductance causes all sorts of weird behavior.
Distributor shaft play, I happened to pull mine out and the shaft would barely spin, this was a reman from Napa. It lasted 18 months. If the shaft touches and grounds out the magnetic reluctor cannot accurately transfer spark, cross firing happens.. this will show on the rotor and cap.
My coil went bad only when it was warm, cold start was good.. saturated coils only rear their heads when temp is an issue.
one way too identify its a bad coil, disconnect the spout plug(timing control for ecu) and if it still acts funny this means its not coming from the ecu, the spark is controlled only by the distributor and coil at this point. This in my opinion is the only way to accurately test those two components.
Click to expand...

Your post sounds like the experiance of somebody that pulled his hair out trying to straighten a small gremlin, which caused a large pain in the a$$. I read it with great interest because I will soon be dealing with a large re-wire project on my truck.
 
If you're absolutely sure your ignition is set up correctly (disconnect the timing bypass wire and set to timing to zero) I would check compression, and all your vacuum hoses are on right. Low compression in on cylinder will cause a shake in one cylinder at idle that smooths out at higher rpm.
 
i have had lots of gm engines not like off brand spark plugs from the oem design/brand of plug .

they develop a skip . change them out for oem brand and style plug and gone went the problem .

but also as said on these old tbi systems look at the cone spray and pattern off each injector with the air cleaner lid off.

if its not a nice smooth cone shape and just dribbles fuel this will cause problems.
 
87sub91 said:
Hi there, I am new to CK5 and I am hoping you guys can help me out with my 1991 4x4 Suburban. The problem that I seem to be having is the truck misfires at idle only. You can hear it in the exhaust and feel the engine shake only when it is idling, put it in drive and it goes away. Drives great with good power and gets decent gas mileage. The truck has the stock L05 350 TBI with 192,*** miles. I have replaced just about everything once or twice. The things I have done to the truck are (new intake gaskets, plugs, wires, dist. cap+rotor, distributor, TBI rebuild, CTS sensor, knock sensor, and map sensor.) I've replaced all the vacuum lines, thinking that the miss was caused by a vac leak, no fix. I have yet to find a vac leak anywhere. I'm just about fed up with this thing.:dunno: Any help will be gladly appreciated, Thanks! Oakley
Click to expand...
 
Try cleaning the air intake and the mas sensor. Remove the mas and other components in the tbi and look for a cracked gasket. I had the same problem with my 03 Explorer. The check engine lamp would come on only at hiway speeds. I had to make a gasket out of solid shelf liner since no gaskets are available any more. That solved the problem.
 
I agree with all the above. The 192,000 should be the tip off, though. Have you checked your spark plugs for oil foul?I think a leakdown test is in order with a small block approaching 200,000 miles.
 
87sub91 said:
Hi there, I am new to CK5 and I am hoping you guys can help me out with my 1991 4x4 Suburban. The problem that I seem to be having is the truck misfires at idle only. You can hear it in the exhaust and feel the engine shake only when it is idling, put it in drive and it goes away. Drives great with good power and gets decent gas mileage. The truck has the stock L05 350 TBI with 192,*** miles. I have replaced just about everything once or twice. The things I have done to the truck are (new intake gaskets, plugs, wires, dist. cap+rotor, distributor, TBI rebuild, CTS sensor, knock sensor, and map sensor.) I've replaced all the vacuum lines, thinking that the miss was caused by a vac leak, no fix. I have yet to find a vac leak anywhere. I'm just about fed up with this thing.:dunno: Any help will be gladly appreciated, Thanks! Oakley
Click to expand...
 
87sub91 said:
Hi there, I am new to CK5 and I am hoping you guys can help me out with my 1991 4x4 Suburban. The problem that I seem to be having is the truck misfires at idle only. You can hear it in the exhaust and feel the engine shake only when it is idling, put it in drive and it goes away. Drives great with good power and gets decent gas mileage. The truck has the stock L05 350 TBI with 192,*** miles. I have replaced just about everything once or twice. The things I have done to the truck are (new intake gaskets, plugs, wires, dist. cap+rotor, distributor, TBI rebuild, CTS sensor, knock sensor, and map sensor.) I've replaced all the vacuum lines, thinking that the miss was caused by a vac leak, no fix. I have yet to find a vac leak anywhere. I'm just about fed up with this thing.:dunno: Any help will be gladly appreciated, Thanks! Oakley
Click to expand...
Also try this to find a vac leak : with the engine warm, not hot or at op. temp. , spray carb cleaner around on the intake, tbi, etc - If there is a change in the rpm or sound , you do have a vac leak caution = only with the engine warm, not hot ♨ a fire or explosion could result. BE CAREFUL !!!
 
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