560SEL Bogging AT Wide Open Throttle
 

Hi Everyone,

I have been working on rough idle and high idle issues for quite a while on my 1991 420SEL with 560SEL motor. I have recently swapped out the fuel injectors with another used set, and I was forced to swap my EHA when I damaged the one that was in the car. Today I replaced idle control valve and it solved both issues almost immediately. I also replaced leaking valve cover gaskets and when I went to test drive the car to check all of the work, I noticed that when I accelerated at wide open throttle, the car seemed as though it was starving for gas. The tachometer crawled to the redline and it felt as though the car was bucking. I have never had that issue before though I have not tested it at Wide Open throttle in some time. I went home and adjusted the EHA to slightly richer. The idle then became perfectly smooth. smoother than it has ever been. I took the car back on the highway and it still was not running well at wide open throttle. It is running great at part throttle. Any ideas as to where I should look?

Thanks as always for your input.

Ever resealed the intake plenum and associated bits? If this has never been done, you need to do it. Before I did this, my 560 would bog going to WOT.

Catalyst could be plugged some too.

Thanks for your reply. It was resealed a few years ago. And the fact that it idles so perfectly now makes me doubt that it is a vacuum leak. Also, the vacuum gauge is alI the way to the left at idle. I thought about the cat also but other than the rough fast idle it was running strong at WOT a few months ago maybe even more recently. My first thought was that the idle control valve that I swapped in caused it but that does not seem possible. Do you know if a bad or improperly adjusted EHA can cause the issue? Bog might not be the right word. If I put my foot down it takes off but as the revs climb it feels like it is starving for gas. I also changed the gas filter and accumulator a few months ago.

The throttle switch has two circuits: Idle & WOT. If the WOT contacts are not closing or there is an open in the circuit, full throttle enrichment will not occur.
Acceleration enrichment will occur due to the rapid swing of the air flow sensor and the potentiometer that is linked to the sensor, but the sustained enrichment will be missing.

Thanks for your reply. How would I check to see if that is the issue?

I found a thread on the switch. I will check it tomorrow.

What exactly is the air flow flap pot for anyway? My '86 420SEL did not have it, all my '89-91s do...

Really hard to test CIS without the proper tools. PM me with your email and I will send you some info Larry at CIS Flowtech was gracious enough to send me.

It was raining yesterday so I could not check the throttle position switch. I tested it today and it is working correctly. Any other thoughts?

With the throttle switch checked, turn to the basics:
Compression check.
Spark plugs.
Ignition wires.
Coil.
Dist cap & rotor.

Is the car still fitted with the 420 ECU, or has one from a 560 been installed?

I have not checked the compression but the spark plugs, wires, cap and rotor are fairly new. will pull a few plugs and see what they look like. The coil is older. The ECU is from a 560. To reiterate, the car has never run smoother since I changed the ICU the other day. Everything is perfect until I try to accelerate hard in the upper RPMs. It will get to the redline but just much slower than it used to and with an occasional buck. I read in another post that throttle linkage is sometimes the culprit. I will check that tomorrow or Monday. I will also check the air filter. It has been a while since I changed it.

I changed a number of things in the last few months trying to get the idle straightened out.
1. injectors ( it just occured to me that I did not change the seals because I did not have new ones handy. Could that be it?)
2. EHA. I damaged the one that was in the car but had a spare (untested) . can the EHA cause this symptom?
3. ICU. Can the new ICU somehow be causing this problem?
4. Fuel filter
5. Accumulator

Where does the signal from the throttle switch go? Could the signal not be making it to its destination?

Could something with the timing advance cause this?

Check fuel pumps for volume, = with pumps electrically jumped to 12 volts via fp relay you need to get 1 litre in 20 seconds, easy to test disconnect main fuel line that goes over valve cove 1 is pressure other return, ive seen
bog when 1 of 2 pumps were found not working

The WOT signal is used by the ECU to a) ignore the O2 sensor value, and b) to provide a fixed-map mixture signal to the EHA. In addition, the WOT signal is used by the ignition controller (EZL) to slightly retard the timing.

What type(s) of spark plug wires, and plugs are installed?

The fuel pumps are definitely a possibility. One was replaced a few years ago but one is old. I thought I heard an intermittent noise coming from the fuel pumps the other day when I went around the back of the car to listen to the exhaust. I will check to see if I am getting the proper volume.

Thanks for the explanation. That helps me rule out the O2 sensor.

The EHA is suspect to me because that is the newest part that was swapped in (prior to the ICU) and it was used. Is it possible that the EHA is the culprit or that is set too lean?

Regarding the spark plugs and wires, It is raining but I am 99% sure that the spark plugs are NGK BP5ES (they could be BP6ES). The wires are the correct Bosch resistor wires. I can get the numbers of the wires later today.

If by "resistor wires" you mean copper conductor w/resistor cartridge at the spark plug connection, then that is correct. If RFI suppression, non-metallic conductor type, not so good; total resistance is likely to be too high.

There is an off chance that the fuel pump relay is the culprit. Since it serves as the rev-limiter, it could be triggering early, OR, mechanical vibration of one of the internal contactors, in sympathy with engine vibration could be interrupting the pump circuit.