Off Idle miss/bog

[naa10104]

Hello,

73 351 C 2V, auto, stock motor except Pertronix ignition and coil, dual exhausts. Freshly rebuilt stock Motorcraft 2100 2 V carb. Car starts, idles, drives fine, no running on or pinging. Have timing a little advanced. Both timing and carb adjusted via vacuum gauge. No vacuum leaks. Have brand new NOS Carb/EGR spacer plate, new EGR and gaskets. Original EGR set up was probably not working as EGR spacer plate was shot and port between exhaust manifold and intake was completely blocked with carbon, all corrected at this point.

When I gradually open the throttle from idle I get a miss/bog which gets a little worse as the RPM's climb. If I rev the engine more sharply ... no bog, sounds good. If I block off the vacuum line at the EGR the miss/bog goes away completely. So, could this miss/bog be a result of lower octane fuel, (run 87 octane) and the advanced timing ? I know I can just block the egr vacuum line and be done with it but I would like to know what the issue is and be able to use the new parts I just installed. THanks for any advice !

 

[jbojo]

Question, when you get the bog is this when driving the car or your indications in the garage and playing with the throttle? There is a speed sensor along with a solenoid which controls this EGR valve so if you haven't driven it try to see if you see the same symptoms on the road. My guess is that this speed sensor is not working or the solenoid valve that it controls vacuum to the EGR valve is bad. That solenoid provides the vacuum to open the EGR and its normally open and the speed sensor commands it shut.

The EGR valve when open is porting some of the exhaust gas back into the intake to be burned again and should only happen when the engine is up to temp and off idle. In 73 they used ported vacuum at idle to keep advance at a minimum and keeping the burn process into the exhaust to reduce emissions. The drawback was higher engine temp at idle and overheating if cooling wasn't adequate or idling long periods in high ambient temp areas.

Vol 6 of the shop manual describes the emissions system operation. I don't have this manual but contemplating on acquiring one.

 

[naa10104]

Question, when you get the bog is this when driving the car or your indications in the garage and playing with the throttle? There is a speed sensor along with a solenoid which controls this EGR valve so if you haven't driven it try to see if you see the same symptoms on the road. My guess is that this speed sensor is not working or the solenoid valve that it controls vacuum to the EGR valve is bad. That solenoid provides the vacuum to open the EGR and its normally open and the speed sensor commands it shut.

The EGR valve when open is porting some of the exhaust gas back into the intake to be burned again and should only happen when the engine is up to temp and off idle. In 73 they used ported vacuum at idle to keep advance at a minimum and keeping the burn process into the exhaust to reduce emissions. The drawback was higher engine temp at idle and overheating if cooling wasn't adequate or idling long periods in high ambient temp areas.

Vol 6 of the shop manual describes the emissions system operation. I don't have this manual but contemplating on acquiring one.

Hello,

Thanks for the reply ... the miss/bog occurs both on the road and in the garage. My car does not have any speed sensor ... very basic vacuum system ... I have looked at many vacuum diagrams and mine appears to be intact. I do have vacuum at the EGR ... When you rev the engine you can see the diaphragm in the EGR moving in and out. The vacuum line that feeds the EGR comes out of the 4- line vacuum connector on the firewall and goes to a tee. One end of this tee goes to the EGR and the other goes to a vacuum switch at the front of the motor near the water pump; believe it is a temperature sensor. Hope that helps.

 

[jbojo]

Hello,

Thanks for the reply ... the miss/bog occurs both on the road and in the garage. My car does not have any speed sensor ... very basic vacuum system ... I have looked at many vacuum diagrams and mine appears to be intact. I do have vacuum at the EGR ... When you rev the engine you can see the diaphragm in the EGR moving in and out. The vacuum line that feeds the EGR comes out of the 4- line vacuum connector on the firewall and goes to a tee. One end of this tee goes to the EGR and the other goes to a vacuum switch at the front of the motor near the water pump; believe it is a temperature sensor. Hope that helps.

The speed sensor is behind the speedometer.

I don't think that there were different emissions differences between cars except for CA. If you have a chance can you post pics of all the emission parts on your motor?

 

[naa10104]

Hello,

Thanks for the reply ... the miss/bog occurs both on the road and in the garage. My car does not have any speed sensor ... very basic vacuum system ... I have looked at many vacuum diagrams and mine appears to be intact. I do have vacuum at the EGR ... When you rev the engine you can see the diaphragm in the EGR moving in and out. The vacuum line that feeds the EGR comes out of the 4- line vacuum connector on the firewall and goes to a tee. One end of this tee goes to the EGR and the other goes to a vacuum switch at the front of the motor near the water pump; believe it is a temperature sensor. Hope that helps.

The speed sensor is behind the speedometer.

I don't think that there were different emissions differences between cars except for CA. If you have a chance can you post pics of all the emission parts on your motor?

hello, can you tell me what the speed sensor looks like ? Been behind the dash a lot and never seen anything that I thought would be a speed sensor ... since I don't what one would look like might have missed it. The only emissions parts on my motor are the EGR valve, and the gas vapor recovery can in the engine compartment. The only sensor is the temperature one that is near the water pump and screws into the front of the block. It has three vacuum nipples ... only two of which have vacuum lines on them, the other is capped.

Made a mistake on what i said about the vacuum lines to the EGR. Correct description is ... vacuum line comes off of the EGR and goes to a tee ... one end of the tee goes to the vacuum switch in the air cleaner which open the flap at the end of the snorkel. The other end of the tee goes to the vacuum/temperature switch at the front of the engine. Thanks

 

[jbojo]

Looks like this, the long end plugs into the back of the speedometer. The speedometer cable from the transmission connects at the other end. That electrical connecter goes to the EGR control module which controls a solenoid to control when the EGR valve is open.

 

[barnett468]

the egr is causing a lean condition.

 

[jbojo]

the egr is causing a lean condition.

It's connected to manifold vacuum so it will always be porting exhaust gas back to the intake. That is where the speed sensor comes in, when the RPM is up leaning the mixture does not have as much of an effect. I still think that some of the emission parts were removed. If the speed sensor is there it will confirm this.

 

[barnett468]

.

he may be missing parts but it runs fine until he plugs the egr in which means the egr is leaning the mixture out because it definitely isn't making it richer . he can leave it unplugged and his problem will instantly be gone or he can do whatever he needs to do to make it run with the egr connected.

 

[naa10104]

.

he may be missing parts but it runs fine until he plugs the egr in which means the egr is leaning the mixture out because it definitely isn't making it richer . he can leave it unplugged and his problem will instantly be gone or he can do whatever he needs to do to make it run with the egr connected.

Hello,

I have had the dash and gauges out of the car several times, unhooking the speedo cable. Never saw that speed sensor, just a very rigid speedo cable that goes into the back of the speedometer. Unless the speed sensor part is on the engine side of the firewall I don't think I have one. If my current set up is leaning out the mixture how could i compensate/correct that ? My understanding is that the two mixture screws on the front of the carb only effect idle (?) If not please let me know. Could it be that I need to retard the timing some and run higher octane as I am running 87 now ? Thanks for all the input from everyone.

Steve

 

[jbojo]

You need to get that EGR off of manifold vacuum or remove the vacuum to it and plug the line. Right now at idle the system is recirculating exhaust gas through the intake. At a minimum you want to use ported vacuum because at idle there is no vac and as you accelerate the vacuum will increase and open the EGR valve after the rpm is up a little. It would help if you can attach pics of the hose routing for the emission stuff to see where the issue might be..

 

[barnett468]

If my current set up is leaning out the mixture how could i compensate/correct that ? My understanding is that the two mixture screws on the front of the carb only effect idle (?) If not please let me know. Could it be that I need to retard the timing some and run higher octane as I am running 87 now ? Thanks for all the input from everyone.

Steve

increase the main jet size.

if the power valve is dried up. replace it.

.

 

[naa10104]

.

he may be missing parts but it runs fine until he plugs the egr in which means the egr is leaning the mixture out because it definitely isn't making it richer . he can leave it unplugged and his problem will instantly be gone or he can do whatever he needs to do to make it run with the egr connected.

Hello,

I have had the dash and gauges out of the car several times, unhooking the speedo cable. Never saw that speed sensor, just a very rigid speedo cable that goes into the back of the speedometer. Unless the speed sensor part is on the engine side of the firewall I don't think I have one. If my current set up is leaning out the mixture how could i compensate/correct that ? My understanding is that the two mixture screws on the front of the carb only effect idle (?) If not please let me know. Could it be that I need to retard the timing some and run higher octane as I am running 87 now ? Thanks for all the input from everyone.

Steve

Hello,

Carb was rebuilt at a rebuild shop in CA and has only been back on the car for a month or so. As far as I know all new components installed and flow tested. EGR and EGR carb/spacer plate all new. If I actuate the throttle with the car off there are two very strong and pronounced squirts of fuel that can be seen in the throat of the carb. Not sure how to check the accel pump ...again can only assume that the carb is good as it was just rebuilt. The fuel in the tank is app. 2 to 2.5 months old 87 octane, but I did put Stabil in the correct amount in with the fuel. The car gets run over the winter on average app. every 7-10 days. If not driven at least run for 15 minutes or so and is in a climate controlled garage. today I put about 9 gallons of 94 octane in and drove it for app. 30 minutes parked for 1.5 hours and then drove another 30 minutes ... no improvement with the gas. No ping or detonation at all ... no run on either.

 

[jbojo]

On your carb front right side looking at it is there a vacuum port near the base, if so is it connected to anything?

 

[barnett468]

your carb is lean with the egr connected, its that simple.

 

[naa10104]

On your carb front right side looking at it is there a vacuum port near the base, if so is it connected to anything?

Yes, vacuum line to the distributor. There is only one vacuum line attachment on my distributor. On the left side of the carb, front base there is a vacuum line that goes to the vacuum/temp switch that is screwed in near the water pump into the block. This switch has three nipples. Line form left carb goes into middle nipple, bottom nipple is capped and top nipple has line from a tee at back of engine ... one side of the tee goes to the EGR the other into the first vacuum connection on the left out of 4 coming out of the firewall. Hope this helps.

 

[jbojo]

OK, so you need to use ported vacuum for the emissions stuff which is the line going to the distributor. Try putting a tee there and run another line to the EGR and plug the current hose that was there. In theory this should turn the EGR off at idle and begin to come on at higher rpm. The ported connection does not provide vacuum at idle so that EGR valve should be closed and not lean out the fuel mixture. Let me know if this works, if not we can think of other solutions.

 

[naa10104]

OK, so you need to use ported vacuum for the emissions stuff which is the line going to the distributor. Try putting a tee there and run another line to the EGR and plug the current hose that was there. In theory this should turn the EGR off at idle and begin to come on at higher rpm. The ported connection does not provide vacuum at idle so that EGR valve should be closed and not lean out the fuel mixture. Let me know if this works, if not we can think of other solutions.

Okay ... I appreciate the response ... and understand ... just to clarify ... the line coming out of the firewall to the T which feeds the egr and vacuum/temp switch, going to removed tee and run only the line directly to the vacuum/temp switch. Will move the tee to the line out of the right side carb and run one side of the tee to distributor and other to EGR. Think that is it ... will let you know what happens, probably won't get to it until Monday ... thanks again.

 

[naa10104]

OK, so you need to use ported vacuum for the emissions stuff which is the line going to the distributor. Try putting a tee there and run another line to the EGR and plug the current hose that was there. In theory this should turn the EGR off at idle and begin to come on at higher rpm. The ported connection does not provide vacuum at idle so that EGR valve should be closed and not lean out the fuel mixture. Let me know if this works, if not we can think of other solutions.

Okay ... I appreciate the response ... and understand ... just to clarify ... the line coming out of the firewall to the T which feeds the egr and vacuum/temp switch, going to removed tee and run only the line directly to the vacuum/temp switch. Will move the tee to the line out of the right side carb and run one side of the tee to distributor and other to EGR. Think that is it ... will let you know what happens, probably won't get to it until Monday ... thanks again.

Hello,

Got to the car today and made the changes you suggested .... the off idle bog/miss is now gone ... road tested also. Thanks. Curious, there is also a port on the passenger side of the carb on the side. A hose runs form this port to the vacuum/temp switch by the water pump ... a second hose runs form another port on this switch to the Bi metal sensor on the under side of the air cleaner to open and close the flap in the end of the snorkel. Are both of these ports on the carb base "ported" vacuum ? Did not see if the flap was working today, but assume it is, will have to check later. What is the correct way to adjust the two idle mixture screws on the base of the carb ? I understand that these only effect the idle mixture and therefore should only be adjusted with the idle very low so that no other fuel circuits are involved. I have heard many different methods to adjust and want to make sure I am doing it correctly. Thanks again for the advice !