Help! Limp Home Mode

[lamarZ06]

..'Limp Home Mode'...85 l98 c4 manual 4/3- starts quick nice idle for about 20 sec. then falls to 'limp home mode' Does not recover...oil pressure switch/sensor.? and or can I disable 'Limp Home Mode'.?.... ALSO : can I remove factory air cond. parts- pull elec. plug vac. hoses but keep compressor until I find a belt and not piss off the ECM.?

 

[Hot Rod Roy]

can I disable 'Limp Home Mode'.?

Why would you want to disable the 'Limp Home Mode'? It would be better to fix the problem! What is the problem?

 

[lamarZ06]

hey roy...

HI ALL...This '85 Doug Nash 4/3... for $1000.oo ...19000 Miles...Motor starts , idles fine/all 8....drops to limp home in about 30 secs.can't find the cause...ac disabled...

 

[Hot Rod Roy]

HI ALL...This '85 Doug Nash 4/3... for $1000.oo ...19000 Miles...Motor starts , idles fine/all 8....drops to limp home in about 30 secs.can't find the cause...ac disabled...

First observation: Your serious lack of understanding of mechanics. Sorry to be so blunt, but this doesn't give us any clue about what you've done, or what you've learned about the problem. What's your fuel pump pressure? How's your ignition? Plugs, cap, rotor, wires? Look at your wires on a dark night. Any visible arcing? A/C being "disabled" has nothing to do with the question. I'll bet money you don't have a Factory Service/Shop Manual (FSM). Get one!

Second observation: If you bought a 19,000 mile '85 for only $1,000, the PO was probably having problems, too. Are you sure that's really 19,000? What did you learn from him? Did you know about the condition when you bought the car? What was your plan?

 

[lamarZ06]

...'limp mode'...

...Hey roy. Thanks for reply..Correction/190,000 mi...'Haynes Repair' and what information I get on internet...What I'm having trouble with is finding the cause of the 'limp home'...Car starts idles fine ..FP to start 40 psi. after 30 seconds FP drops to 20/15 psi...hit the throttle..or try to roll 1st. gear/load.. straight to limp mode...car has not been diagnosed...What I need to know 'is there any way to eliminate this function from the ecm'.....don't know how many owners... but the car has been raped. wires cut.. wiper motor in a box..no head lites, no ac no windows etc. I gave 1000 and drove it home...but the computer has taken over and shut it down...so I'm also trying to find someone mobile to read the alcl...I'm not going to start throwing money at it...yet...If there was no 'limp home' car could be at least be brought back to life...thanks for your time...j. I've tried to by pass oil pressure switch but doing that closed the switch not allowing full power to the fuel pump...I'm tempted to run dc to the pump like after I check my fire insurance...lamar "Thanks Again Roy"

 

[Hot Rod Roy]

'Haynes Repair' and what information I get on internet

The Haynes is almost useless. Too much missing, and even more, misleading information. With a car in the condition of yours, you'll definitely appreciate the FSM!!

FP to start 40 psi. after 30 seconds FP drops to 20/15 psi.

The first thing I'd try would be a new fuel filter.

I've tried to by pass oil pressure switch but doing that closed the switch not allowing full power to the fuel pump.

This statement makes no sense to me. The oil pressure switch applies 12 v. to the fuel pump when the engine is running and the oil pressure is above 4 psi. What voltage do you see on the fuel pump? What do you mean when you say "not allowing full power to the fuel pump"?

 

[lamarZ06]

FSM check.....

..also to the best of my knowledge the oil pressure switch is what the ECM uses to regulate or trip the 'limp home'.... j.

 

[lamarZ06]

Fsm...

...also Helms list shop man. @ $82///////////owners $32... j.

 

[Antz81]

Sounds like you've got a lot of work ahead of you.

A haynes manual is not worth the paper it's printed on compared to the FSM. Search on ebay for one. You won't mind the extra expense once you see how detailed they are. An AIM might not be a bad idea for your car either.

if you search the knowledge base section you should be able to find how to read any fault codes stored in the ECM. You won't need any special tools for this.

 

[Hot Rod Roy]

..also to the best of my knowledge the oil pressure switch is what the ECM uses to regulate or trip the 'limp home'.... j.

That's the second statement that makes no sense to me. Where did you get this info on a "limp home mode"?

Here's the diagram for your '85 fuel pump circuit. If the engine is running, the oil pressure switch keeps 12 v. on the fuel pump, and the fuel pressure should stay high. If the fuel pressure goes that low, the engine will definitely stumble. That's not "limp home", that's a poor running engine!!

(Click on picture to enlarge)

Unless you answer my questions, I won't be able to help you.

 

[lamarZ06]

...Thanks Antz...

...Is your '81 an L98 tpi w/ countless sensors feeding an ECM.? and.. I don't know the AIM.? Thanks.. j. this c4 may never be more than a hot rod but so far I can't get past the 'limp home'.. lamar

 

[Hot Rod Roy]

...Is your '81 an L98 tpi w/ countless sensors feeding an ECM.?

Lamar, it sounds like you're trying to insult Antz. I surely hope I misinterpreted that statement! I'd appreciate a better understanding of this comment, if I'm wrong!

 

[lamarZ06]

...thanks...I've been going over these circuits for a couple of months...daily .. j

That's the second statement that makes no sense to me. Where did you get this info on a "limp home mode".

Here's the diagram for your '85 fuel pump circuit. If the engine is running, the oil pressure switch keeps 12 v. on the fuel pump, and the fuel pressure should stay high. If the fuel pressure goes that low, the engine will definitely stumble. That's not "limp home", that's a poor running engine!!
Most info via internet.j something is regulating the voltage to the pump... I was thinking it was the oil pressure switch the only circuit break between bat. and pump. At start the pressure gauge reads 40 psi. drops to 20/15 psi. within 30 secs.
(Click on picture to enlarge)[/QUO TE]

 

[Antz81]

...Is your '81 an L98 tpi w/ countless sensors feeding an ECM.? and.. I don't know the AIM.? Thanks.. j. this c4 may never be more than a hot rod but so far I can't get past the 'limp home'.. lamar

My 81 is stil running the L81. Not as many sensors as the L98, but to make things more confusing the ECM is controlling a carb.
The AIM is the assembly manual. Very handy for working out how things should be. It's been an invaluable reference guide for me.

I don't think the problem you are seeing is a limp home mode. It sounds more like low fuel pressure. Which could be a few different things. Fuel filter, fuel pump, Fuel pressure regulator.

 

[lamarZ06]

...thanks...

My 81 is stil running the L81. Not as many sensors as the L98, but to make things more confusing the ECM is controlling a carb.
The AIM is the assembly manual. Very handy for working out how things should be. It's been an invaluable reference guide for me.

I don't think the problem you are seeing is a limp home mode. It sounds more like low fuel pressure. Which could be a few different things. Fuel filter, fuel pump, Fuel pressure regulator.

..I suspect the FPR..filter new..I'm not ready to pull the plenum...not ready to throw money at it until I'm able to isolate problem..pump new...Fuel pressure held 40 psi. for 50 miles then went to limp... it never recovered,,,fuel tank drained before new pump some water...I need to check voltage at pump before and after LHM.....Thanks again for your help...lamar

 

[lamarZ06]

...#$%^&...

Lamar, it sounds like you're trying to insult Antz. I surely hope I misinterpreted that statement! I'd appreciate a better understanding of this comment, if I'm wrong!

'''don't know how you could possibly interpret my question as an insult.. surely was not my intention'''

 

[lamarZ06]

...fpr...

...QUESTION...if the fuel pump regulator was damaged; could it produce 40 psi. at the schrader then drop to flutter at 20/15 psi.? it will idle at 20/15 ruffly then die ...thanks ANTZ j.

 

[Antz81]

...QUESTION...if the fuel pump regulator was damaged; could it produce 40 psi. at the schrader then drop to flutter at 20/15 psi.? it will idle at 20/15 ruffly then die ...thanks ANTZ j.

Not sure.
if you turn the ignition on but don't start the fuel pump shouldrun for 2 seconds. After that does the pressure stay up or does it drop back down?
there's two checks that can be done on the FPR.
Disconnect the vacuum line to it and make sure no fuel comes out.
The other check (can reconnect vacuum line before this) is to clamp the return line (believe this must be done at the tank) and see if the pressure drop continues. Be aware when you do this the pressure could (and most likely will) go higher than 40PSI.

 

[lamarZ06]

...thanx...

Not sure.
if you turn the ignition on but don't start the fuel pump shouldrun for 2 seconds. After that does the pressure stay up or does it drop back down?
there's two checks that can be done on the FPR.
Disconnect the vacuum line to it and make sure no fuel comes out.
The other check (can reconnect vacuum line before this) is to clamp the return line (believe this must be done at the tank) and see if the pressure drop continues. Be aware when you do this the pressure could (and most likely will) go higher than 40PSI.

...I do find the fpr suspect, I have pulled the vac no fuel found...I'm almost convinced that's it but not enough to pull the plenum...I checked the voltage at the tank... the positive feed to the harness looked poor ....reconnected,pump running i was able to shut down the motor with the slightest touch....I'm going after the harness =...I have yet to replace a harness though that has solved any problem....if this fails return lines, fuel canister, sensors go and the EPA can kiss my ass...it should be against the law to strangle a detroit v8...2 4's set of Webbers and I'll be skipping that #$%^ computer across the river...thanks again Antz

 

[Antz81]

Sounds like that poor connection at the pump could be the cause of your problem.