Bosch Fuel Injection Miss and Rough Acceleration

fastfiat81

Mr. Speed
1986 with stock fuel injection 700 miles on the new engine. I’m working through an issue that has cropped up. The car has been running flawlessly until this point. The engine idled and accelerated super smoothly. Then I washed and waxed the car. I didn't hose down the engine bay or even open the engine cover. Took it out a day later and it has a miss and doesn’t accelerate smoothly. Died at idle.

I have replaced the coolant temp sensor with a new Bosch unit. The wiring checks out between the sensor and the ECU. All FI grounds are good. Cleaned the FI ECU, TPS and MAF connectors with deoxit. I pulled the distributor cap. All looks good. Plug wires are tight. Cleaned the reluctor and connector with deoxit. Spark plugs look perfect. Fuel pressure is a constant 36 psi.

I even rebuilt the Bosch ICM with a GM HEI module (always wanted to do this) and replaced the coil with a MSD Blaster 2 and removed the resistor.

The car still has a slightly rough idle and a good miss on acceleration, but it doesn’t die at idle now. I am going to reset the timing later this week. I’m coming to the end of things I can think of. Anyone have any thoughts?

Thanks-
 

Jonohhh

True Classic
Is it a dead miss on one cylinder (consistent rhythm) or a random, multi cylinder misfire that changes with RPM or load? That can help narrow things down since the list of possible causes for a general misfire is quite extensive.
 

fastfiat81

Mr. Speed
Not really sure. The miss is at any rpm. I'm going to put a test light on all injector connectors to see what is happening.
 

DaveR

True Classic
Since you washed the car, check under the engine cover on the passenger side to see if the drain over the wheel well is open. If water is retained in that area it can cause issues.
 
How old are your plug wires? If they are resistance wires, they can go bad in a few years. You can check the resistance with a DVM. They generally should be under about 2,000 ohms. Getting them wet wouldn't help if they were marginal to start.

I'd also check out @DaveR's comment on the passenger side drain. That is where the Bosch ignition box is and it is known to get submerged if that drain gets plugged.
 

Rod Midkiff

True Classic
I hate to have this thought. I read it's a fresh engine,,,, pull the oil cap and make sure you have oil in the top end (the head gasket can be installed backward and plug off the oil feed. (I hope I am just blowing smoke on this)
 

Dan Sarandrea (Phila)

Waitin' On Parts...
1986 with stock fuel injection 700 miles on the new engine. I’m working through an issue that has cropped up. The car has been running flawlessly until this point. The engine idled and accelerated super smoothly. Then I washed and waxed the car. I didn't hose down the engine bay or even open the engine cover. Took it out a day later and it has a miss and doesn’t accelerate smoothly. Died at idle.

I have replaced the coolant temp sensor with a new Bosch unit. The wiring checks out between the sensor and the ECU. All FI grounds are good. Cleaned the FI ECU, TPS and MAF connectors with deoxit. I pulled the distributor cap. All looks good. Plug wires are tight. Cleaned the reluctor and connector with deoxit. Spark plugs look perfect. Fuel pressure is a constant 36 psi.

I even rebuilt the Bosch ICM with a GM HEI module (always wanted to do this) and replaced the coil with a MSD Blaster 2 and removed the resistor.

The car still has a slightly rough idle and a good miss on acceleration, but it doesn’t die at idle now. I am going to reset the timing later this week. I’m coming to the end of things I can think of. Anyone have any thoughts?

Thanks-
The only data point you supplied that can be analyzed is wrong. " Fuel pressure is a constant 36 psi. "

Operating fuel pressure should be very close to 28 psi. The only time fuel pressure should hit 36 psi is when the vacuum line to the fuel pressure regulator is disconnected or pinched off (as in during the FP test here https://xwebforums.com/wiki/images/4/43/Fi1.jpg )

If the vac line to the regulator is leaking, you could have a weird set of circumstances combining false air (unmetered air allowed into the intake) with overfueling from high fuel pressure.

The FI manuals says to also check for fuel return line blockages.
 
Last edited:

kmead

Glutton for punishment
Most likely an ignition issue. A miss would be very unlikely to be an FI issue.

An engine needs fuel, spark and to be in time. That you have fuel enough to rev up suggests that both fuel and timing are good. A miss would be most likely related to a bad plug, bad wire, an issue with a distributor component (rotor, cap or unlikely the Hall sensor) or in the control system.

Check your distributor cap, does it still have the carbon button hanging down to contact the center of the rotor?
Check each plug wire to see if that is the issue.
Recheck the change you made to the ignition module, the modules do go bad and then there is the loose nut doing the wire crimping (not casting aspersions, just one of those things that can happen particularly in a high vibration environment.
Check for water ingress into the case for the GM chip.
Intermittent ground to the ignition system?
 

fiatfactory

Steve Cecchele
Have you retorque the cylinder head since rebuild?

6-700 miles is about typical for a failure with no retorque.

Do a compression test, usually lifts between 2 and 3.

SteveC
 

fastfiat81

Mr. Speed
First of all thanks to everyone for their comments so far. What I have done to date is the following:

I have replaced everything in the ignition system. Rebuilt ICM with a NOS GM ignition module, New Bosch cap and rotor, new spark plugs and plug wires, new MSD blaster 2 coil. I ohmed out the reluctor/ hall and it tested good. I verified that I have spark on each cylinder plug. I rebuilt this distributor when I restored the car, but I'm going to pull it again and have a close look.

I have checked all grounds and they look good. I even added new terminals to the FI harness grounds. All ground pods look good. I went through the FI diagnostic manual and everything checks out there. I swapped out the AFM and ECU to no positive effect.

The injector connectors have 12V when the key is on. The engine drops in RPM for each cylinder that I pulled the injector connector when running. I have triple checked all hoses and sprayed starter fluid on all connections including the intake manifold so 'm pretty sure I don't have a vacuum or air leak in the system.

Compression tests shows: 155,150,155,155 on 1-4 cylinders respectively. No head gasket re-torque as I have TTY bolts and a Felpro gasket.

Fuel pressure is correct as noted in my first post. I'm leaning towards dirty/clogged fuel injectors as the culprit. Either that or the distributor. In the next few days I'm going to pull the entire fuel system and send the injectors off to be cleaned. They have only 700 miles on them since being rebuilt so my tank must be a source of the problem. It looked pretty darn good when I checked it, but I plan to clean it out with phosphoric acid. I'll replace the pre and post filters and flush the system hoses as well. I'm also going to check the intake and exhaust manifold nuts/bolts for proper torque.

This is my plan of attack unless someone has any other ideas. I doubt valve clearances will be the problem, but I'll check those before I take out the injectors.

Thanks all-
 
Last edited:

kmead

Glutton for punishment
First of all thanks to everyone for their comments so far. What I have done to date is the following:

I have replaced everything in the ignition system. Rebuilt ICM with a NOS GM ignition module, New Bosch cap and rotor, new spark plugs and plug wires, new MSD blaster 2 coil. I ohmed out the reluctor/ hall and it tested good. I verified that I have spark on each cylinder plug. I rebuilt this distributor when I restored the car, but I'm going to pull it again and have a close look.

I have checked all grounds and they look good. I even added new terminals to the FI harness grounds. All ground pods look good. I went through the FI diagnostic manual and everything checks out there. I swapped out the AFM and ECU to no positive effect.

The injector connectors have 12V when the key is on. The engine drops in RPM for each cylinder that I pulled the injector connector when running. I have triple checked all hoses and sprayed starter fluid on all connections including the intake manifold so 'm pretty sure I don't have a vacuum or air leak in the system.

Compression tests shows: 155,150,155,155 on 1-4 cylinders respectively. No head gasket re-torque as I have TTY bolts and a Felpro gasket.

Fuel pressure is correct as noted in my first post. I'm leaning towards dirty/clogged fuel injectors as the culprit. Either that or the distributor. In the next few days I'm going to pull the entire fuel system and send the injectors off to be cleaned. They have only 700 miles on them since being rebuilt so my tank must be a source of the problem. It looked pretty darn good when I checked it, but I plan to clean it out with phosphoric acid. I'll replace the pre and post filters and flush the system hoses as well. I'm also going to check the intake and exhaust manifold nuts/bolts for proper torque.

This is my plan of attack unless someone has any other ideas. I doubt valve clearances will be the problem, but I'll check those before I take out the injectors.

Thanks all-
Given how the issue manifested itself after water was applied to the car, I would think it best to go back through and look carefully at all ignition and FI connections looking for water ingress and continued grounding of a wire.

I recall in the past some folks having an issue where water migrated along the injection system harness and eventually found an area in a wire where it grounded either within the harness sheathing or at the main connector to the ECU. Checking for continuity of each wire in the harness from the main connector to the actual sensor etc although time consuming and tedious, costs very little.

I know this is a long shot but it has happened.
 
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