Engine surging at operating temperature

[matrophy]

86 720 carbureted with z24 with 68000 original miles. Online searching for this problem showed results for newer cars with a lot more gadgetry on the engines so I could use some 720 specific troubleshooting.

 

I replaced a broken wire on temp sensor and ran the engine in the driveway to warm it up and check  that the gauge was working.

Engine idles fine.

Once I approached operating temperature, I revved the engine to get it to heat up faster and once I got it to operating temperature the rpms surged between 1500 and 2000 rpm

 

I ran out of daylight so today I started the engine up and the same thing happened. Good idle - surging between 1500-2000 rpm but not until it reached operating temp.

Haynes manual says to check:
1) Vacuum leak. Didn't find one.

2) Fuel pump = I have a pressure/vacuum gauge but am not sure how to use it with a live fuel line.

3) Exhaust side coil not firing - checked and it seems OK

 

I have EGR blocked off.

According to the PO the choke wasn't working and his mechanic fixed it open..

I had a problem recently with throttle sticking with linkage sticking on the carb.

I lubed it but not thoroughly and  I remember reading a thread on a similar subject here where a sticking carb linkage might be the culprit of surging at idle.

 

I haven't been able to drive the truck since I discovered this so I'm not sure if this is something that happens on the road - I assume so but my truck is blocked in and I won't be able to move it until some construction is finished here. 

 

Any suggestions for what might be the cause?

[datzenmike]

You say good idle but no numbers, in the same sentence 1,500 to 2,000 so what does surging mean to you???

 

What's it idling at?  

 

What carburetor? Two wire or round connector with 6 wires?

 

 

Won't be fuel pump. All it does is keep the carburetor full.

[matrophy]

Idle with no pressure on accelerator is at around 800 rpm. Surging only happens when trying to hold 2000 rpm or above. When it reaches 2000 rpm it drops to 1500 rpm then back up to 2000 rpm and so on. Pretty sure it is round connector with 6 wires.

[datzenmike]

That's an ECC carburetor. It's half carburetor and half EFI, a kind of cross over. It uses a solenoid for the primary barrel the rapidly opens and closes many times a second.It's controlled by a computer under the drivers seat with RPM, air temp, water temp, throttle position and feedback from an O₂ sensor in the exhaust to keep the mixture near stochiometric.

 

These ECC (electronic controlled carburetor) are known to have broken wires (from flexing) at the round plug. Move the wiring around while revved up, see if the surging goes away. Try to find which one is loose.

 

 There are are voltage and continuity checks that can be done to the computer to check the sensor inputs. Later Z24i engines with TBI have a diagnostic test mode that will tell you what's wrong. Too bad, this doesn't also. 

[matrophy]

I see that the ECCS doesn't kick in until the engine is at operating temp so now I wonder if my reattaching the broken wire on the temp sensor is what caused this to start.  I'll check the continuity of the connections at the plug. I'll also run the truck to operating temp and disconnect the temp sensor to see if that stops the surging. That may help narrow down the cause.

[datzenmike]

When cold the choke will be on so there's no ECC during warm up. The O₂ sensor is in open loop and goes into closed loop control when the water sensor (NOT temp gauge) warms up above 122F IF not during a start, not full throttle heavy load, not decelerating, battery voltage is above 10v.

[matrophy]

30 minutes ago, datzenmike said:

When cold the choke will be on so there's no ECC during warm up. The O₂ sensor is in open loop and goes into closed loop control when the water sensor (NOT temp gauge) warms up above 122F IF not during a start, not full throttle heavy load, not decelerating, battery voltage is above 10v.

I see. I'll check the plug for continuity and go from there.

[matrophy]

I jiggled and twisted the connector repeatedly with engine running while holding the throttle linkage at around 2000 and the surging continues. Up and down from approximately 1500 to 2000 rpm. Do you think it might be the O2 sensor? I saw in a google search that a couple of people only had the problem when the vehicle wasn't driving - just when parked. I haven't been able to drive the truck but maybe the ECC senses that the transmission is engaged and this is only an issue when its not driving? 

Edited November 13, 2020 by matrophy

[datzenmike]

Replacing random parts without proof is expensive and time consuming. That way lies madness.

[matrophy]

2 minutes ago, datzenmike said:

Replacing random parts without proof is expensive and time consuming. That way lies madness.

 

I wouldn't replace anything without being certain that's what it was. When I was in my teens it was a different story. 

[datzenmike]

I have an '84 FSM that details the voltage checks at the ECU under the seat. This would pinpoint the problem or faulty sensor. Only problem is the year. If you can buy an '85 FSM that should work. Beware ordering anything '86, it was a part year and may show up as a D21 Hardbody manual. I was also under the impression that all 720s built after April '85 might be Z24i with the fuel injection engines.

[matrophy]

I have an 86 FSM (marked 1986.0) that shows instructions for checking components of the ECCS. I plan on going through the complete diagnostic procedure for the ECCS as detailed in the FSM and go from there.

 

Even though I doubt now that the o2 sensor is the culprit, I probably won't be able to do the test of the o2 sensor by reading the pulses on the control module because I can't maintain 2000 rpm. Do you know if the indicator will pulse after running at less than 2000 rpm for 2 minutes?

 

Would the fuel cutoff system activating when it is not supposed to cause the surging on and off?

 

  Mine was built in 5/85 and is carbureted. I generally use 85 for parts and 84 for wiring as the 84 diagram matches up with my wiring perfectly. Pretty sure I have California emissions on it except I don't have the air induction tubes. My truck seems to be a bunch of stuff from different years thrown together at the end of the 86 model year. Like the Johnny Cash song...

[datzenmike]

The tests are mostly continuity of wiring and that grounds are good. There is a throttle valve switch adjustment. 

[matrophy]

I was able to drive the truck today and had no issue with the surging when driving. Before I took it for a drive, I found out that the ground wires (one on valve cover and one on exhaust manifold) between the engine and the firewall were loose where they attach to the firewall and I cleaned up the terminals and retightened them and check for continuity. I was really hoping that somehow that was the cause of my surging problem but no luck.

 

When it's sitting in the driveway, revving to 2000 rpm causes the engine to surge back and forth between 1500 and 2000 rpm. With the clutch pedal in and the shifter in gear while parked- no surging. I know my neutral safety switch isn't working but I am confused about how that would affect the engine. The ECCS does receive an input from the NSS so maybe?

[datzenmike]

I believe there is a fuel shut off during deceleration. Naturally if coasting in neutral the engine would stall and if shifting gears the clutch being in would stall the engine. In both cases the NSS and the clutch switch would  disable the fuel shutoff. I know this is not when deceleration but maybe it thinks it is.

 

Have you checked the throttle position switch? If decelerating the throttle would be at idle, no??? You are revved to 2,000 but if the switch is at fault the ECC may be being told it's at idle.

[matrophy]

My next step is to check all of the remaining inputs to the ECCS. The fuel cut-off info shows that there is some cycling between 1800 and 1500 rpm at temp. I'm not clear if the fuel shut-off instantaneously causes the rpms to drop. The instructions tell how to check the circuit but I'm not sure how to determine if the switch is bad. The instructions also say to hold at 2000 rpms which might be difficult given the surging

 

[matrophy]

Checked every component of ECCS according to FSM instructions (except for Spark Plug Switching Control) and all were in spec and are working how they are supposed to except for Neutral Safety Switch which I have on order. I will check the Spark Plug Switching Control tomorrow but I don't think that is relevant.

[datzenmike]

Spark plug switching is done by a vacuum switch that senses low vacuum heavy load condition. The is sent to the distributor module that shuts off the exhaust side plugs and also increase the spark advance timing to compensate for the single plug operation.

[matrophy]

I was sure it was the Throttle Valve Switch but that is functioning like it is supposed to. I'll check the Spark Plug Switching system and wait for the NSS to arrive. Maybe that's what is causing the surging.

 

On another related subject. I thought the choke was adjustable by rotating the spring housing  like what I remember about Holly carbs. Mine is riveted on so I assume that it is opened and closed with a linkage somewhere. Apparently it's been fixed open by the PO and I would like to get it working again since the cold weather is coming. I haven't really looked at it and I wanted to ask how I could go about getting it to function again. I did check and it doesn't look like there is power to the heater terminal with ignition on so I have to check the relay and fuse which I'll do tomorrow

[datzenmike]

Yeah they used to be adjustable but with emissions becoming more and more stringent the choke is set at the factory and they don't want some idiot owner turning the richness up.

 

What ever the PO did, undo it. The choke should close when cold and be fully open in 8-10 min of driving. My guess is the choke does not open and turn off and rathere than fix it he just jammed it open. See what happens and report back.

[matrophy]

Apparently I don't have vacuum switch 'C' to check the Spark Plug Switching circuit. The footnote says it's for non-California engines and my VIN says this is a Calif engine. Either that or it is in a different location but I looked at everything with a vacuum hose coming out of it and didn't see anything that looked like a vacuum switch that I didn't already know what it was.

 

I double-checked timing and idle speed and  the mixture setting on the carb. While it was running I operated the throttle from the engine bay and you can definitely hear a clicking around the carb when the rpms reach 1800-2000 which I assume is the fuel cutoff. The rpms drop and climb back to 2000-ish before cutting off again and hearing the click. I'm assuming that's a symptom and not a cause at this point. 

 

As for the choke, I checked the fuse and the relay clicks when the key is turned on. The clutch is always open but can be closed manually but a spring in the shaft opens it right up. It wasn't jammed open in other words. I attached a photo below with a mirror showing the best shot of the linkage I could get to see if you notice anything missing.

 

I didn't get a voltage reading between the choke heater wire and ground with the key on but I want to check it again. I also saw somewhere about checking continuity between the heater wire and the carb body but haven't done that yet.  I should have checked if I can feel warmth on the carb heater after it ran a while. I'm thinking that even if the heater was working it wouldn't explain why it doesn't close on it's own so I'm thinking maybe there is a linkage piece missing?

 

[datzenmike]

If a California engine it will have an EEC carburetor with a 6 wire round electrical plug, and a O₂ sensor in the exhaust manifold. 

 

 

 

MODEL (3rd line down) If California it will have a V as or near the last letter. If Federal emissions it will have a U

[matrophy]

That's what I have so no Spark Plug Switching system then.

 

 

 

 

[matrophy]

Model number is NL720STVF so that confirms it's a Cal emissions vehicle.

[matrophy]

Reporting back: I installed the new neutral safety switch and after I did I tried it out and the truck will still start in gear so I was worried that the old "no good"  switch may not have been what was causing the surging. I started up the truck and let it get to operating temperature and there was no surging. Not sure why but I'm ready to move on. I did some bench testing of the old and new switches and the old switch had no continuity across the leads where the new switch had continuity at all times. I'm not sure if this means that the new NSS is no good. The switch is supposed to have continuity when the shifter is in neutral and an open circuit when in gear. Testing the circuit of the new switch shows continuity in all cases. I am a little worried that if the new NSS is actually defective that the problem may come back when driving since there is supposed to be an open circuit when in gear. I'll check it out on the road tomorrow.