Help! Dtc 55

[John Robinson]

I keep getting a DTC 55 when I am wot over 70 MPH. I have replaced the in line filter, the fuel pump, removed the screen from the regulator and I have the following fuel line pressure numbers

Car warmed up and at idle 41# with vac off pressure regulator

33# vac on engine idling.

33# @ 2500 RPM with the vac on

41# @ 2500 RPM with the vac off

Engine off and holding 40# now for over 10 min.

Somebody help me out here because I think I should have more than 33# at 2500 RPM. If so what else should I be checking?

 

[TedC]

John, This may only apply to TPIs. In a recent article posted on tuning TPIs, it was mentioned that there should be a 1 PSI drop for every 2 inches of vacuum. If you apply the same logic (could be wrong), then 41-33 is an 8 PSI drop which suggests 16 inches of vacuum as the source. My 1989 L98 is 44-36 or a drop of 8 PSI.

 

[TedC]

Found this...
DTC 55 will set when Engine is operating in Closed Loop,
not in fuel cutoff, (no DTCs 44, 45, 64, 65 set). If the PCM detects a lean oxygen sensor voltage for 3 seconds during power enrichment modes of operation, DTC 55 will set.

Attach a fuel pressure tester to the fuel rail. With the engine running, the pressure should be around 41-47 psi.

Turn the engine off. The fuel pressure should VERY slowly bleed off.

Some of the causes could be a bad fuel pump or clogged strainer or clogged fuel filter.

 

[gmjunkie]

I keep getting a DTC 55 when I am wot over 70 MPH. I have replaced the in line filter, the fuel pump, removed the screen from the regulator and I have the following fuel line pressure numbers

Car warmed up and at idle 41# with vac off pressure regulator

33# vac on engine idling.

33# @ 2500 RPM with the vac on

41# @ 2500 RPM with the vac off

Engine off and holding 40# now for over 10 min.

Somebody help me out here because I think I should have more than 33# at 2500 RPM. If so what else should I be checking?

You have a Kinked Line,Restriction,Plugged F Filter,Faulty Fuel Pressure Regulator or Faulty Fuel Pump John!!
LT1/LT4 Engines
You should have 41-47# at key up!
3-8# less Idling,and the fuel pressure should build up from there as the load increases!!

I like to see them at least 38-39# Idling with vacuum!

 

[Hib Halverson]

First thing I do is stop WOT testing if DTC55 keeps setting. The OBD is telling you the engine is lean at WOT and if you keep running it like that you're asking for a fried piston or a burned out cat.

One problem you have is your fuel pressure, 40 psi with the engine off is, as the Junk Man suggests, too low. That alone might account for the engine lean at WOT.

If you had no problem with the filter or the pump. Like Junk says, look for restrictions in the system. Also, there's a ball check in the fuel system around the pump somewhere. I can't remember if it's built into the pump or is in the pick-up assy. but, if that ball check is leaking, you'll have inadquate fuel delivery.

Because your fuel pressure is a bit low, I'd check the fuel supply system, first, before I'd go looking for bad injectors.

Also, this entire discussion assumes both your O2Ses are functioning properly.

 

[gmjunkie]

I can't remember if it's built into the pump or is in the pick-up assy. but, if that ball check is leaking, you'll have inadquate fuel delivery.

It's in the pump Hib!!

 

[John Robinson]

The Pump

OK this is the second new pump and I had this problem with the pump I took out and now with the new one. Is the check valve in the dampener?

Also the static fuel pressure held 40# for 30 min and after 6 1/2 hours I still had 20# of fuel pressure.

As for the dampener when I put the new pump in it seemed to slide up and down the tube fairly easy as I remember it. Should it be tight on the tube like a vacuum line?

 

[Hib Halverson]

(snip)

Also the static fuel pressure held 40# for 30 min and after 6 1/2 hours I still had 20# of fuel pressure.

Perhaps you missed the point in several previous posts. If so, let's be clear...

It doesn't matter if the pressure holds for 2 minutes or 2 days, at 40 psi your fuel pressure is TOO LOW.

If you look at the Factory Service Manual, you will see that the specification is 41-47 psi. It is likely that the DTC55 is being caused by some problem in the fuel supply system. It's also possible that once the engine runs at WOT for a second or so that fuel pressure is dropping far more than 1 psi below the minimum. Fuel flow has to drop off a lot for DTC55 to set.

If you don't already have a copy, I'd obtain the FSM for 1993 and read that book's significant amount of troubleshooting info for low fuel pressure and DTC55.

Problems could be

Faulty fuel pump
Wrong fuel pump
Restricted fuel pump strainer
Leaking fuel pump feed hose.
Restricted fuel hose or pipe
Restricted fuel filter
Wrong fuel filter

Lastly if DTC55 is set but DTC44 or 64 are not, the problem is very likely in the fuel supply system.

Good luck.

 

[John Robinson]

New information

Today I pulled the pump and worked with the Dampner. In the kit I got with the new pump was a sleeve to connect the pump to the fuel feed line for some other model. I too that piece and lid i tup the fuel line and then put the dampner on. Nest I lide the extra connector back tight against the dampner to hold it in place.

Now the fuel pressure reading are as follows

Vacuum on regulator 37#

Vacuum removed from the Regulator 47#

Started the car and went around the block it seemed a lot more responsive to the throttle. Let it sit and cool off and just now went for a 8 mile drive. Treated it gently for 4 or 5 miles and checked the temp it was at 205 so I gave it a try and it was flat and I had an SES light until I got off it. Drove a short distance and hit it again and it rin like it should and no SES light.

I am in a waiting game now to see what it will do the next time I drive it.

 

[Schrade]

Today I pulled the pump and worked with the Dampner. In the kit I got with the new pump was a sleeve to connect the pump to the fuel feed line for some other model. I too that piece and lid i tup the fuel line and then put the dampner on. Nest I lide the extra connector back tight against the dampner to hold it in place.

Now the fuel pressure reading are as follows

Vacuum on regulator 37#

Vacuum removed from the Regulator 47#

Started the car and went around the block it seemed a lot more responsive to the throttle. Let it sit and cool off and just now went for a 8 mile drive. Treated it gently for 4 or 5 miles and checked the temp it was at 205 so I gave it a try and it was flat and I had an SES light until I got off it. Drove a short distance and hit it again and it rin like it should and no SES light.

I am in a waiting game now to see what it will do the next time I drive it.

Don't wait. Stick the paper clip in, and see what the 'H' History code is.

 

[John Robinson]

HCode

Don't wait. Stick the paper clip in, and see what the 'H' History code is.

Paper clip was sacrificed for the advancement of knowledge. I had only one H code and it was 55. That does not surprise me since I hav'nt cleared the codes in several days I was getting the SES light at least once a day so I don't really know how old it was. I cleared it and now I will have to wait and see if it codes again. Dont worry Hib about my burning a valve because at my age (Almost 70) I am burning enough nitro to burn my own valves.If I could put my nitro in my car it probable would make it to the moon. WOT is better than anything else I can do at my age.

 

[Schrade]

Paper clip was sacrificed for the advancement of knowledge. I had only one H code and it was 55. That does not surprise me since I hav'nt cleared the codes in several days I was getting the SES light at least once a day so I don't really know how old it was. I cleared it and now I will have to wait and see if it codes again. Dont worry Hib about my burning a valve because at my age (Almost 70) I am burning enough nitro to burn my own valves.If I could put my nitro in my car it probable would make it to the moon. WOT is better than anything else I can do at my age.

Sub - lingual nitro tabs?

Put a couple in the tank UH hUH They WILL boost the squeeze

 

[John Robinson]

This is getting ugly

Got the car out for a trip today. It ran good until I got to the I slab and after a few miles I had the SES again. The next thing it did was to stumble or act like it was hitting a rev limiter when I tried to accelerate. (NOT WOT). This symptom came on when it started to rain. When the rain stopped I got the SES again but this time the car performed like it had full power. The code it stored was a DTC 64. I am getting frustrated and cannot figure out what is prompting this. Could the ground to the ECM cause this behavior? O as a side note my mileage has gone into the toilet.;help;help;help

 

[Schrade]

Mileage is tanked because DTC's have defaulted the fuel trim to max.

If you have a code, solution is cake. Follow diagnostic procedure in FSM. Since a 55 code is invalid if there's a 64, like TedC posted,

Found this...
DTC 55 will set when Engine is operating in Closed Loop,
not in fuel cutoff, (no DTCs 44, 45, 64, 65 set).

then all out assault DTC 64 per FSM.

edit:
looks like it's almost workin' ITSELF out for ya' uh huh .

 

[John Robinson]

Wires

I found the ground wire to the tps broken and repaired it this morning. I will have to drive it a ways to get the ecm to reset itself. It looks like the car was running in closed loop all the time. It will be interesting to see how it performs when it goes into open loop. I would think that the MPG will improve now as well.

 

[So Bay Vetter]

Out of curiosity, where was the ground break ie: in the tps plug, harness or?

 

[John Robinson]

Wires

Out of curiosity, where was the ground break ie: in the tps plug, harness or?

The break was in the plug. A while backI visited a salvage yard and went through several wireing harness gathering plugs and wire along with some dash light bulbs in there sockets. I only gathered them from GM cars so that the connectors I got might be direct replacements for connectors in the car. The purpose of that was to have a collection of different colors, wires and connectors. From that collection I was able to salvage a piece of wire and the plug end and put them into the TPS socket and then splice the wire into the harness to the TPS.

As a side note from one of the dash light sockets and bulbs I salvaged I made a continuity tester. For some reason I do not yet understand that tester is polarity sensitive. One wire is always Positive and one is negative and they will not work in the reverse order.:cool

 

[gmjunkie]

I found the ground wire to the tps broken and repaired it this morning. I will have to drive it a ways to get the ecm to reset itself. It looks like the car was running in closed loop all the time. It will be interesting to see how it performs when it goes into open loop. I would think that the MPG will improve now as well.

I think you mean it was running in OPEN LOOP Jon!!

And not going into Closed after warm up!!

 

[John Robinson]

O Yeah

Caught again in another Senior moment.

Quick update had the car out today and it is a lot more response and pulls stronger even on moderate throttle. I am going to baby it for a few days to give everything time to readjust itself. When I first started it up and put it in gear it idled rough but as soon as I drove it up the street it straightened out and as soon as it got up over 180F the idle smoothed out and the exhaust note sounds healthy and strong. I have my fingers crossed that this is the end of this problem. Thanks to all who chimed in and kept me thinking and looking at all the possible solutions to this problem.

 

[John Robinson]

Confused

I have been trying to get my brain around the closed loop and open loop description in the fsm. The explanation for loop status is as follows

"Closed Loop" displayed indicates that the ECM is controlling fuel delivery according to oxygen sensor voltage. In "Open Loop," the ECM ignores the oxygen sensor voltage and bases the amount of fuel to be delivered on TP sensor, engine coolant, and MAP sensor inputs only.

So until the engine gets up to temp it is operating in open loop and that is the only time the TPS, or the Engine Coolant and the MAP sensors have any input into the ECM. That is contrary to what I would have thought because it would seem to me that the ECM would need to know what the throttle demand was and how would it know that from the oxygen sensor.

Could someone help me understand what is going on here in 3 volumes or less.